The windowFeatures parameter is available on WidgetMessage: tm-open-external-window
I would be keen to be able to at least try windowFeatures="location=no, toolbar=no"
on the WidgetMessage: tm-open-window because as I am opening tiddlers, the wikis address is not useful. And adds clutter to the window.
tm-open-window is defined in $:/core/modules/startup/windows.js
and
tm-open-external-window in $:/core/modules/startup/story.js
strWindowFeatures = paramObject.windowFeatures;
window.open(strUrl, strWindowName, strWindowFeatures);