Proof of concept for my symmetrical keyboard.

swapkeys.c

/*
 * Based on https://renenyffenegger.ch/notes/Windows/tools/swap-keys_c
 * This code uses Windows' hooks to catch and redirect input.
 * This code is not yet tested for DLL compilation.
 */
#include <stdio.h>
#ifdef __CYGWIN__ // 2017-07-18 (tsettgchabe)
#define _WIN32_WINNT 0x0500 /* 0x0404 */
#endif
#include <windows.h>

HHOOK hook;
#ifdef AS_DLL
#pragma message("is DLL")
int i = 0;

BOOL APIENTRY DllMain (HMODULE hModule, DWORD reason4call, LPVOID lpvReserved)
{
	switch (reason4call) {
	case DLL_PROCESS_ATTACH:
		MessageBox(0, "Hey there!", "SwapKeys", 0);
		break;
	case DLL_THREAD_ATTACH:
	case DLL_THREAD_DETACH:
	case DLL_PROCESS_DETACH:
		break;
	}
	return TRUE;
}
#endif

#define LLAYER (1 << 0)
#define RLAYER (1 << 1)
static int layermode = 0;

LRESULT CALLBACK keyboardHook(int nCode, WPARAM wParam, LPARAM lParam)
{
	KBDLLHOOKSTRUCT *p = (KBDLLHOOKSTRUCT *)lParam;
	DWORD newVkCode;
	INPUT inputs[3];
	UINT ret;

	if (nCode < 0)
		return CallNextHookEx(hook, nCode, wParam, lParam);

	char vkStr[2];
	vkStr[0] = p->vkCode;
	vkStr[1] = 0;
	printf("%3lu | %9s | %3d\n", p->vkCode, vkStr, p->scanCode);

	int iskeyup = (wParam == WM_KEYUP || wParam == WM_SYSKEYUP);

	inputs[0].type = INPUT_KEYBOARD;
	inputs[0].ki.wScan = 0;
	inputs[0].ki.dwFlags = iskeyup ? KEYEVENTF_KEYUP : 0;
	inputs[0].ki.time = 0;
	inputs[0].ki.dwExtraInfo = 0;
	inputs[0].ki.wVk = 160;

	inputs[1].type = INPUT_KEYBOARD;
	inputs[1].ki.wScan = 0;
	inputs[1].ki.dwFlags = iskeyup ? KEYEVENTF_KEYUP : 0;
	inputs[1].ki.time = 0;
	inputs[1].ki.dwExtraInfo = 0;
	
	inputs[2].type = INPUT_KEYBOARD;
	inputs[2].ki.wScan = 0;
	inputs[2].ki.dwFlags = KEYEVENTF_KEYUP;
	inputs[2].ki.time = 0;
	inputs[2].ki.dwExtraInfo = 0;
	inputs[2].ki.wVk = 160;

	if ((p->flags & LLKHF_INJECTED) == 0) {
		int input_count = 1;
		switch (p->vkCode) {
		// Tab 9 -> L-Layer
		case 9:
			if (wParam == WM_KEYUP || wParam == WM_SYSKEYUP) {
				layermode &= ~LLAYER;
			}
			else {
				layermode |= LLAYER;
			}
			return 1;
		// [ 219 + ] 221 -> R-Layer
		case 219:
		case 221:
			if (wParam == WM_KEYUP || wParam == WM_SYSKEYUP) {
				layermode &= ~RLAYER;
			}
			else {
				layermode |= RLAYER;
			}
			return 1;
		// CapsLock 20 -> L-Ctrl 162
		case 20:
			inputs[0].ki.wVk = 162;
			break;
		// B 66 -> B 66, \ 220
		case 66:
			if (!layermode)
				goto quit;
			inputs[0].ki.wVk = 220;
			break;
		// J 74 -> J 74, [ 219
		case 74:
			if (!layermode)
				goto quit;
			inputs[0].ki.wVk = 219;
			break;
		// K 75 -> K 75, ] 221
		case 75:
			if (!layermode)
				goto quit;
			inputs[0].ki.wVk = 221;
			break;
		// L 76 -> L 76, ' 222
		case 76:
			if (!layermode)
				goto quit;
			inputs[0].ki.wVk = 222;
			break;
		// M 77 -> M 77, { S219
		case 77:
			if (!layermode)
				goto quit;
			if (iskeyup) {
				inputs[0].ki.wVk = 219;
			}
			else {
				inputs[0].ki.dwFlags = 0;
				inputs[1].ki.wVk = 219;
				input_count = 3;
			}
			break;
		// N 78 -> N 78, | S220
		case 78:
			if (!layermode)
				goto quit;
			if (iskeyup) {
				inputs[0].ki.wVk = 220;
			}
			else {
				inputs[0].ki.dwFlags = 0;
				inputs[1].ki.wVk = 220;
				input_count = 3;
			}
			break;
		// L-Win 91 + L-Ctrl 162 -> ESC 27, Tab 9
		case 91:
		case 162:
			inputs[0].ki.wVk = layermode ? 9 : 27;
			break;
		// R-Ctrl 163 -> Win 91
		case 163:
			inputs[0].ki.wVk = 91;
			break;
		// ; 186 -> ; 186, " S222
		case 186:
			if (!layermode)
				goto quit;
			if (iskeyup) {
				inputs[0].ki.wVk = 222;
			}
			else {
				inputs[0].ki.dwFlags = 0;
				inputs[1].ki.wVk = 222;
				input_count = 3;
			}
			break;
		// , 188 -> , 188, } S221
		case 188:
			if (!layermode)
				goto quit;
			if (iskeyup) {
				inputs[0].ki.wVk = 221;
			}
			else {
				inputs[0].ki.dwFlags = 0;
				inputs[1].ki.wVk = 221;
				input_count = 3;
			}
			break;
		// ' 220 + \ 222 -> R-Ctrl 163
		case 220:
		case 222:
			inputs[0].ki.wVk = 163;
			break;
		// OEM102 226 -> L-Shift 160
		case 226:
			inputs[0].ki.wVk = 160;
			break;
		default:
			goto quit;
			break;
		}
		ret = SendInput(input_count, inputs, sizeof(INPUT));
		return 1;
	}
quit:
	return CallNextHookEx(hook, nCode, wParam, lParam);
}

#ifdef AS_DLL
// 2017-07-21 - tsettchabe / CYGWIN
extern __declspec(dllexport)
#endif
int startSwapKeys()
{
	MSG messages;

	hook = SetWindowsHookEx(WH_KEYBOARD_LL, keyboardHook, NULL, 0);

	if (hook == NULL) {
		printf("Error %d\n", GetLastError());
		return 1;
	}
#ifndef AS_DLL
	printf("Waiting for messages ...\n");

	while (GetMessage (&messages, NULL, 0, 0)) {
		TranslateMessage(&messages);
		DispatchMessage(&messages);
	}
#endif
	return 0;
}

#ifndef AS_DLL
int main(int argc, char *argv[]) {
	return startSwapKeys();
}
#endif