マルチチャンネルモードでチャンネル全てを選択するスクリプト

It works when like in your example there are no default Color Mode Channels (so like for ex. RGB):

sTT = stringIDToTypeID; for(i = 0; i < (cnl = activeDocument.channels).length;) {
	(ref = new ActionReference()).putName(sTT('channel'), cnl[i++].name);
	(dsc = new ActionDescriptor()).putReference(sTT('null'), ref)
	dsc.putBoolean(sTT('extend'), true), executeAction(sTT('select'), dsc)
}