Is there a way to use custom segments as restrictions on roles, just like in-built segments?

In the custom segment record, you can define which roles can have permissions to access it. In the Role page, Custom Segments is a permission under Setup. What kind of specific scenario are you thinking of?

We have to restrict the access of a role (Project Manager) to instances of Bills. We want a Project Manager user to see only those bills that belong to projects that the user leads or bills that are assigned to him/her to review. We thought about segments as an option, but any other suggestion is welcomed!

For that specific scenario a custom segment might be overkill but not sure..My suggestion is you can try to create a Saved Search for the Vendor Bills with a criteria like Project field value = Name of employee. Check the Available as List View. Then go to the PM role and under Searches tab>Standard, pick Transaction as Type and in List View find the SS you created then check the Restricted checkbox. I tried it quickly and it seemed to work but do some testing like searching for a bill in the global search for the PM role and if the restriction also affected the list view of other records.

Thanks a lot for your help! We were trying something similar already, so your advices will be very helpful!