Set Custom Registration Point with JSFL?

Here is a command that changes the registration point of a selected symbol to the trasformation point (that was set manually in Free Transform Tool by dragging the pivot), while keeping the symbol in the same place on the stage. The command adjusts the position of all the elements inside the symbol, on all the layers and in all the keyfames:

var doc = an.getDocumentDOM();
var tl = doc.getTimeline();
var lib = doc.library;
var sel = doc.selection;
var cl = tl.currentLayer;
var cf = tl.currentFrame;

if (sel.length > 0) {
	for (s = 0; s < sel.length; s++) {
		if (sel[s].elementType == "instance" && sel[s].instanceType == "symbol") {
			var sym = sel[s];
			var symsel = 1;
			break;
		}
	}
	if (symsel == 1) {
		doc.selectNone();
		sym.selected = true;
		var tp = doc.getTransformationPoint();
		var tpx = tp.x;
		var tpy = tp.y;
		var mat = sym.matrix;
		var sx = sym.x;
		var sy = sym.y;
		var rnd = Math.floor(Math.random() * 1000);
		doc.selectNone();
		tl.addNewLayer("REFREC", "normal", false);
		doc.setFillColor("#FF0000");
		doc.addNewRectangle({
			left: sx - 50,
			top: sy - 50,
			right: sx + 50,
			bottom: sy + 50
		}, 0);
		var recnm = "REC_" + rnd;
		var refnm = "REF_" + rnd;
		tl.setSelectedFrames(cf, cf + 1);
		doc.convertToSymbol('graphic', recnm, 'center');
		doc.convertToSymbol('graphic', refnm, 'center');
		var ref = tl.layers[cl + 1].frames[cf].elements[0];
		doc.enterEditMode('inPlace');
		var reftl = doc.getTimeline();
		var refrec = reftl.layers[0].frames[0].elements[0];
		refrec.x = tpx;
		refrec.y = tpy;
		doc.exitEditMode();
		ref.matrix = mat;
		doc.selectNone();
		ref.selected = true;
		doc.breakApart();
		var rec = tl.layers[cl + 1].frames[cf].elements[0];
		var newx = rec.x;
		var newy = rec.y;
		doc.deleteSelection();
		tl.deleteLayer(cl + 1);
		sym.selected = true;
		doc.setTransformationPoint({
			x: 0,
			y: 0
		});
		sym.x = newx;
		sym.y = newy;
		doc.enterEditMode();
		var stl = doc.getTimeline();
		stl.expandFolder(true, true, -1);
		var lz = stl.layerCount;
		stl.setLayerProperty('locked', false, 'all');
		for (i = 0; i < lz; i++) {
			var lay = stl.layers[i];
			if (lay.layerType == "folder") {
				continue;
			}
			var fc = lay.frameCount;
			for (f = fc - 1; f > -1; f--) {
				var kf = lay.frames[f].startFrame;
				f = kf;
				var fe = lay.frames[kf].elements[0];
				if (fe) {
					fe.x -= tpx;
					fe.y -= tpy;
				}
			}
		}
		doc.exitEditMode();
	}
} else {
	alert("No Symbol Selected");
}

>>... Is this even possible with JSFL alone?

If your question is whether it is possible for the registration point to have custom coordinates set with X and Y during the creation of the symbol, the answer is no, it is not possible. The method document.convertToSymbol() only accepts some of these predefined strings as arguments. However, you can change the registration point of the symbol immediately after its creation, which is entirely within the capabilities of JSFL.

To do this, open the symbol for editing after creating it and move its content to align with the registration point as desired. Use document.enterEditMode() and document.exitEditMode().