It's always allowed, Zenphoto is under the GPL, so the only requirement is that you license any released code under the GPL as well.
However, I don't like the idea of different hacked versions of zenphoto floating around, as it makes support harder and is more inconsistent all around. I'd much rather you send the code to me and I'll do my best to integrate it. Of course, since you have it done already, and these are features people might want, I wouldn't be totally against it. :-)