This was a previous idea by @SharkDesign. It didn't get any support but I think it's a good idea.
The problem with using iPart files with a Custom property is that all of the member files generated are stored in the Workplace root folder. This can lead to hundreds of files, which I'm sure you can appreciate is a right pain in the nads.
There was a suggestion that setting up a library location & adding a proxy folder would solve the issue but I've tried that & it doesn't work. Both James & myself have tried to find a current solution to this problem but it doesn't seem to exist.
So please Autodesk, have a look at giving the option to store the multitude of files generated in a single, definable location, or even the same for standard iPart member files (in a sub-folder local to the iPart file location.
Hopefully, this will get some support.