That’s a reasonable goal. Right now things are only set up to be able to do that running under Node.js. There are more pieces that would need to be joined together by a TW developer to be able to do it in the browser.
Right now, the problem is that external text tiddlers automatically load themselves as soon as you open the tiddler to view them. That turns them into an ordinary tiddler for the purposes of the next save of the wiki. The fix would be a way to restore the external-text setting to any modified tiddlers when saving the wiki.
You can experiment by navigating to
http://tiddlywiki.com/, and then turning off your wifi/network before clicking on the link to “Alice in Wonderland”. You’ll see a tiddler tagged with "external-text” and a message that the content is being loaded.
Best wishes
Jeremy.