I tried pulling from your git repository but I haven't seen any
updates.
The best way to do this would be to look at src/mobius/bootstrap.slate
and looking at what files it loads. Then make a file that loads all
those files after the image has bootstrapped. One issue is that some
of those files have "bootstrap fixups" which should be moved into
separate files so that loading the files twice doesn't cause any
problems.
The location of the source is given by the lexer, so when you evaluate
code in your environment, you might want to fill out that field with
something that your editor can understand and find.