i have had success with plugging the brass bypass hole then redrilling with a smaller hole.also i caught up with a mate the other day and while dialling a cam for him we got to talking on this subject.he was running a so called "proper" ???? cleveland thermostat that looked like a normal brass one but had a small extension that came from it with a ring of steel and a rubber seal around this ring.so when it got to op temp the ring would come down and seal off bypass completely.he does alot of main street idle cruising in summer and has not had any problems since he fitted this thermostat,which got recomended to him by someone else that came across it.
i hope this post makes some sense to you if not i will try to get some pics of it, if no one else can shed some light on this thermostat.
the ONE thing i know for sure is: no matter how big and nasty YOUR CAR is, someone always has one thats BIGGER and NASTIER. build a bridge and get over it i did.