Yep TextAdept never really got mush visibility, but it was (is?) a great approach to a having a editor fully scripted in lua, much more so than Scite which also has lua scripting and of course is uses the same scintilla text editor component as TextAdept.
These days Adobes Brackets is using pretty much the same approach in the JS side of things.
The main issue is that Github do not have CORS support for http access to repos (though they do support CORS for access to repos via their proprietary API) and I don't think the other big public hosters like Bitbucket or Google Code support CORS either.
On the subject of big pack files, sure you'd want to stream them to storage instead of trying to store in memory (for instance JGit has this as an option) but I agree Tim that to start with, even supporting small-medium size repos would be a huge win.
These days Adobes Brackets is using pretty much the same approach in the JS side of things.