Hi Andrew -
Contacts and custom Lists cannot be linked to Releases, as you've seen. A Customer wouldn't "belong" to a Release the same way a Defect or Requirement would.
We suggest setting up a separate Custom List for your Release information. This would contain the full, identifiable Release Name and any other details about that Release that you might want. Then on the Contact or Customer List, create a custom field based on the Release Name list, perhaps another custom field for Date Delivered or Date Installed - whatever other details you might need.
Hope this helps!