Ok i had a few thoughts pop into my head while reading this.
First off, as said before, there already is stealing in trammel. Believe it or not, alot of trammel players (at least on atlantic) have become used to having their guilds infiltrated. They have started to adapt, and are developing ways to prevent these thieves from robbing their guild. Fallon Valor can back me up on this, noting the past instances between P0O and UWF. This adaptation proves that a form of PvP stealing CAN (and already does) exist in trammel without a mass amount of players quitting UO.
That being said, onto the suggestion.
As said by another poster, give thieves jail time in trammel. Allow stealing, but with the following consequences:
If you steal in trammel and are guard whacked, rather than just having the thief die, have him/her teleported (equivalent to being "squelched" by a GM) to the new town jail. When teleported, any uninsured/unblessed items the thief is carrying are placed in a box near the jail cell, freely lootable to all. The stolen item, upon guard whacking, is automatically returned to the victim (which already is the case if guard whacked). Also, allow teleporting to jail when killed in a guard zone by another player (when you steal you are freely attackable by all if you turn criminal).
FYI, a thief who attempts to steal at a crowded bank (even at 120 Stealing) has a very high chance of being noticed. If a thief turns grey (criminal) after attempting to steal in a guard zone, and an NPC (example: the minter) is a few tiles away, the NPC will automatically call guards, resulting in an insta-whack for the thief.
Here's where the fun comes in, especially for you roleplaying trammies. When a thief is thrown in jail he/she is automatically held for 24 hrs, in which time a trial by judge/jury must take place. A player recognized as a judge may sentence thieves to a maximum of one rl week in prison, depending on the severity of the theft.
This may seem like a stretch, but i assure you it isn't, considering i've already been brought before a UO player-run court, which of course was very biased and found me and my guildmate guilty. Afterwhich, we laughed, opened a gate to fel, and said, "Come get us." They followed, and we proceeded to kill them. I believe it was a large blacksmith guild on atlantic.
After being found inevitably guilty, the thief is teleported back to jail for the set amount of time. While in prison, a thief can still talk in all chats, but cannot recall, nor use the physically stuck help menu.
Player "guards" must keep watch, taking turns around the clock. A thief cannot lockpick the cell himself, but an outside thief can. If a thief is able to escape, then they must "lay low" for the remainder of their sentence. They are freely attackable until the sentence is over (which isn't much of a change since we're already freely lootable in trammel...which is ridiculous considering we aren't supposed to be able to steal from trammies, but they can loot us). If spotted in a town by an NPC, then guards will be called, automatically returning the thief to jail.
If another thief attempts to lockpick a cell, they will instantly turn criminal, allowing a nearby player-guard to click them and place them in jail as well, to await a court hearing.
Visitors, as well as onlookers, are allowed to visit thieves in jail. Maybe they'll even play a game of dice through the cell.
Thieves who attempt to steal in trammel will know of these consequences, and agree to them the moment they steal an item.
This court system would see old thieves returning, as well as a new group looking to be law-enforcers in UO. Thieves are happy, trammel players can exact revenge on thieves (rather than having to sit by and watch as their guild is infiltrated, robbed, then left by a thief looking to make a quick buck), and insurance is left alone...everyone wins.
To any nay-sayers, you can already be robbed in trammel without a thief having to worry about being guard whacked. If this idea is implemented, will you quit UO, or adapt? I think you'll adapt. And if you feel so strongly to seeing thieves punished, become a UO law-enforcer and send us to prison, guarded around the clock by other volunteers.