Your comments

Instead of explicitly implementing support for individual vcs, one could implement a WebDAV client connecting to an underlying vcs.

After a short look in the net I've found that WebDAV can serve git and svn, but the status of mercurial support is unknown.
The idea is not bad... Perhaps Textastic could parse over the text and fill the scratch pad with comment lines starting with
   - ' TODO '
   - ' FIXME '
or whatever...