main.js

/* global Compartment, trace */

export default function main() {
  let g1Value = 'g1 original value';
  const g1Slot = {
    get p1() {
      trace(`getting g1: ${g1Value}\n`);
      return g1Value;
    },
    set p1(v) {
      trace(`setting g1: ${g1Value} -> ${v}\n`);
      g1Value = v;
    },
  };
  const c1 = new Compartment("mod", { g1Slot });
}

manifest.json

{
	"include": "$(MODDABLE)/examples/manifest_base.json",
	"modules": {
		"*": [
			"./main",
			"./mod",
		],
	},
	"preload": [
	],
}

mod.js

trace(globalThis.g1Slot.p1 + '\n');
globalThis.g1Slot.p1 = 'new value';
trace('mod done.\n');

LOG:

getting g1: g1 original value
g1 original value
setting g1: g1 original value -> new value
mod done.