I make themes for .com first and then sell them on .org. I add things to the themes, before selling them on .org, but they are all added through hooks and filters so that I only have to maintain a single theme.
I think I could make it compatible with the guidelines. I think the only real requirement is the structure of the credits. I imagine they're using a regular expression to find and replace the credits info (since the credits are dynamic on .com).
In fact I've just checked the underscores footer and it differs to mine anyway
The footer I use is here:
I'm going to update the granule footer to allow customisation (and make it a bit closer to underscores).
For reference this is the version of the footer before the changes:
UPDATE: I've changed the footer now so that we can filter the footer content. If you return true then you can hide the default footer and replace it with whatever content you like through the filter. Not technically how filters are meant to work but does the job