Deploying customized installs

I have about 15-20 instances of a php/mysql project i'm working on. all of them use the same basic framework but each project can potentially have slight customizations to the code. if i make modifications a framework file (say the install script or something), is there a good way to deploy that update to the 15-20 installs, without destroying the customizations? Manually merging those updates into the projects is becoming a little too much work. btw, my live instances are just subversion working copies, so something that involves a svn update projX or something like that would work great