Do you have a vision how it all should look like?
Do you want a list of forms, created by end-users, to be displayed on the separate browse screen in a table with all their attributes? Do you want to use a generic filter, for example, with these entities? If so, it will be a bit problematic to implement this feature.
Or it will be enough if there is only “Process instances” screen that displays all process instances? Different dynamic forms are displayed when users click process action button, and all the data related to this process instance is stored in process variables and process attachments.
There is another way you can go. Probably you’ve heard about it. Products built on CUBA can be easily extended. You can use your product as a base project for client projects. Each of client projects can contain entities necessary for this client only. We wrote about it in the blog: https://www.cuba-platform.com/blog/2015-07-17/421