Possible solution (that is somewhat understandable to the players) for the issue with bypassing claim shield by a villager inside the party:
As only rights that the character actually has can be bled into the party, just make the ability to bypass the shield by siege engines being a sole property of the lawspeaker (in his role of him being the claim 'owner'). Then a lawspeaker could party up with people to enable them to do such things, but a simple villager can't - so the mechanic stays similar enough to personal claims to avoid confusion while removing these exploits.
Given the contents of the video in the thread I linked, this should imho also apply to changing locks (only abused for griefing anyway).