I've not tried it yet in TC11.3, so you'll have to test it. I've seen a few issues when adding custom properties to projects, but it changes version to version.
1. BMIDE deploy fails if TC_Project ACL is modified. Must change ACL back to OOTB to deploy template.
2. Custom properties are read-only. Sometimes this one can be resolved by reassigning the TC_Project OOTB stylesheet back to TC_Project.
3. Custom properties can only be edited by Project Administrator.
Hopefully this helps.
Make sure to "Set the Initial Values to NULL" for the custom property that you define on TC_Project
Otherwise, you will not be able to create anymore new projects.