I've got something that is pretty much exactly what you want here without any kind of RDBMS backend.

While I didnt write the core functionality, i use it and it works really well just using the FS. The application was written only using core modules so it is easily transportable. It was written extensible enuff to add new object types easily (ie document, photo, mp3, others). You can add all your fields to each leaf node.

Its also got a fully indexed search engine which uses a hash it brings up off the disk (not a tied hash). Its also smart enuff, so when creating the index it does an incremental build.

I guess what i'm saying is, you may not really need a database.. :-)