Is there documentation on how to build an altcoin with gitian and/or how to adapt bitcoin's contrib/config files to use gitian with altcoins?
No. But you may freely use and clone
Pfennig, it's the latest Bitcoin forked and moved to scrypt, fully tested, and it gitian builds. I released it freely exactly for this requirement. It will also be updated to each tagged bitcoin release, and also merge upstream tagged releases from Bitmark, so you get the best of all worlds.
Check the commit history to see the applicable changes you may need to make to another code base.
Hint if it's for a fork of an old codebase/clone you are going to have significant work, you may be better to apply any changes to Pfennig, if you do each feature as a pull request then we could build a repository that enables any coin developer to apply patches to the base code and configure a safe modern alt coin. Instead of the existing old rubbish.
Best regards, and if you need assistance with forking pfennig I will do what I can to help.
