Addon to Galv's Message Busts plugin to support using facesets when no bust can be found.

GALV_MessageBusts_SubEmpty_Addon.js

// GALV_MessageBusts_SubEmpty_Addon.js
 /*:
@plugindesc v0.1 Addon to Galv's Message Busts plugin to support using facesets when no bust can be found.
@author Decanter

@help
Place directly after Galv's Message Busts in the plugin order, as this addon directly overrides several methods. Be sure both plugins are somewhere after Yanfly's Message Core, if using that.

How it works: If the game tries to load an image - any image - that doesn't exist, it will load an invisible placeholder image instead (overriding the engine's normal behavior, which is to just throw an error message and crash). Then, in the context of deciding whether to show a face or a bust, the game basically asks "is this bust a real image or a 0x0 placeholder image" and if it's placeholder then it will show the face.

Full disclosure: I developed this plugin for a particular project I liked - Once Ever After, an adult game by Sierra Lee - and only tested it against that project's use cases.

Terms of use: Just give me credit. If you modify the plugin, make a note of that.
*/

// OVERRIDE - Will replace errored bitmap with empty bitmap.
ImageManager.isReady = function() {
    for (var key in this._cache) {
        var bitmap = this._cache[key];
        if (bitmap.isError()) {
            // throw new Error('Failed to load: ' + bitmap.url);
            this._cache[key] = this.loadEmptyBitmap();
        }
        if (!bitmap.isReady()) {
            return false;
        }
    }
    return true;
};

// OVERRIDE
Game_Message.prototype.setFaceImage = function(faceName, faceIndex) {
	switch (faceName) {
		case 'PartyLeader':
			var faceName = $gameParty.leader().faceName();
			break;
		case 'PartyMember':
			if ($gameParty.members()[faceIndex]) {
				var faceName = $gameParty.members()[faceIndex].faceName();
				var faceIndex = $gameParty.members()[faceIndex].faceIndex();
			} else {
				var faceName = "";
			};
			break;
	};
    
    tempName = faceName + "_" + (faceIndex + 1); // using function arguments, be careful
    Galv.MB.ImageChecker(tempName);
    
    Galv.MB.Game_Message_setFaceImage.call(this,faceName,faceIndex);
};

// OVERRIDE
Window_Message.prototype.drawMessageFace = function() {
	var name = $gameMessage.faceName() + "_" + ($gameMessage.faceIndex() + 1);
    Galv.MB.ImageChecker(name);
	if (!$gameSystem.bustDisableTemp) {
        // belatedly change x offset since it's normally decided too soon for ImageChecker
        this._textState.x = this.newLineX();
        this._textState.left = this.newLineX();
        return;
    }
	Galv.MB.Window_Message_drawMessageFace.call(this);
};

// NEW
Galv.MB.ImageChecker = function(name){
    $gameSystem.bustDisableTemp = $gameSystem.bustDisable || false; // renew per-line bust status
    
    if ($gameSystem.bustDisableTemp == false) {
        var img = ImageManager.loadPicture(name + Galv.MB.f);
        if (img.height <= 1) {
            $gameSystem.bustDisableTemp = true;
        }
    }
    return;
};

// OVERRIDES
if (Galv.MB.prio == 0) {
// UNDER MESSAGE

	Window_Message.prototype.newLineX = function() {
		if ($gameSystem.bustDisableTemp) {
			return Galv.MB.Window_Message_newLineX.call(this);
		} else {
			return 0;
		};
	};
	
} else {
// OVER MESSAGE

	Window_Message.prototype.newLineX = function() {
		if ($gameSystem.bustDisableTemp) {
			return Galv.MB.Window_Message_newLineX.call(this);
		} else if ($gameMessage.faceName() && Galv.MB.prio == 1 && $gameMessage._positionType == 2 && $gameSystem.bustPos == 0) {
			return $gameMessage.bustOffset;
		} else {
			return 0;
		};
	};

};

// OVERRIDE
Sprite_GalvBust.prototype.loadBitmap = function() {
	var name = $gameMessage.faceName() + "_" + ($gameMessage.faceIndex() + 1);
    
	if ($gameSystem.bustDisableTemp) {
		var img = ImageManager.loadPicture('');
	} else {
        var img = ImageManager.loadPicture(name + Galv.MB.f);
    };
    
	if (img.isReady()) {
		if (this.bitmap) {
			//this._destroyCachedSprite();
			this.bitmap = null;
		};
		this.bitmap = img;
		this.name = name;
        this.hasBust = true;
	};
};

// OVERRIDE
Sprite_GalvBust.prototype.controlBitmap = function() {
    // if image changed...
	if ($gameMessage.faceName() && this.name !== $gameMessage.faceName() + "_" + ($gameMessage.faceIndex() + 1)) {
    	this.loadBitmap();  // reload bitmap
	};
	
	if (Galv.MB.msgWindow.openness <= 0 || !this.hasBust || $gameSystem.bustDisableTemp) {
		this.opacity = 0;
		this.name = "";
		this.hasBust = false;
		return;
	};

	if ($gameSystem.bustMirror) {
		this.scale.x = -1;
		var offset = this.bitmap.width;
	} else {
		this.scale.x = 1;
		var offset = 0;
	};

	this.opacity = $gameMessage.faceName() ? Galv.MB.msgWindow._openness : this.opacity - Galv.MB.fadeOutSpeed;
	
	// Control image position
	switch (Galv.MB.msgWindow.tempPosType) {
	case 0:
		this.y = this.baseY();
		break;
	case 1:
	//top and middle
		this.y =  this.baseY() - Galv.MB.msgWindow.y;
		break;
	case 2:
	//bottom
		if (Galv.MB.prio == 1) {
			this.y = Galv.MB.msgWindow.height - this.bitmap.height;
		} else if (Galv.MB.pos === 1) {
			this.y = this.baseY();
		} else {
			this.y = this.baseY() - Galv.MB.msgWindow.height;
		};
		break;
	};
	
	if ($gameSystem.bustPos == 1) {
		// if on the right
		if (Galv.MB.prio == 1) {
			this.x = Galv.MB.msgWindow.width - this.bitmap.width + offset;
		} else {
			this.x = Galv.MB.msgWindow.x + Galv.MB.msgWindow.width - this.bitmap.width + offset;
		};
	} else {
		// else on the left
		if (Galv.MB.prio == 1) {
			this.x = 0 + offset;
		} else {
			this.x = Galv.MB.msgWindow.x + offset;
		};
	};
};