How often do lenders actually enforce a due-on-sale clause between co-owners?

Graham, you are treading in dangerous waters, I would advise against this. Keith Jean-Pierre Managing Principal The Dapper Agents Real Estate Operations in ALL 50 States

I would be cautious about anyone saying lenders rarely enforce this. There is no reliable published enforcement rate for your exact situation, and continued payments do not guarantee protection. Being an existing co-owner and exchanging no money do not automatically exempt the transfer. If you are legally married, spousal protections may apply. A quitclaim deed transfers ownership but does not remove your partner's loan obligation. Before recording it, have a Pennsylvania real estate attorney review the deed and mortgage, and obtain written confirmation from the loan servicer that the transfer is permitted. That gives you much more certainty than relying on the likelihood of enforcement