You might take a look at the Museum Archive software project: http://www.musarch.com/
My local history society is looking at converting our Excel inventory listing to it. I don’t know that it is a true open-source system where you can modify the code but it seems pretty flexible so you may not need that anyway. The basic edition is free and the premium edition requires purchase of the developer’s book on the system (about $25 on Amazon). It’s a pretty basic system if that is adequate for your needs.
