Discoveries:
1) It's pretty easy to jump out of the Monastery. There's a good spot if you go over to the fighting arena thing.
2) It's possible to leave the monastery and finish chapter 1 as a Dongsman, but it's screwy and you probably shouldn't. I jumped out of the monastery, went to not-Khorinis and got the guy to give me the deliver-a-package quest to the Dong, who gave me the negotiate-with-inquisitor quest that ends the chapter. Went back, the guy at the front door says, "What the hell are you doing? Get back in the monastery" and cutscene-forces you back in. You can then go around and talk to people and they are all misfiring their scripts between you being a monastery guy and you being a Dongsman.
If you go to talk to the inquisitor, the guys guarding him won't let you into his room and the intended solution for a Dongsman (their script lets you through) doesn't happen. I felt very clever by tricking this using the create-a-prostitute spell to distract the guards, then slipping past them without the script firing. The inquisitor guy gets mad at you for entering his room, but you can just lure him to the door to talk to him, which lets you finish the quest and advance to chapter 2, wherein all seemed well.
Except I went back to the Dong, and he won't sell me any patented chapter-2-of-a-Gothic-game armor upgrade and I didn't get the monastery version from the 'quizitor. Looking online for the problem it seems I "should" have been able to get the armor from the Dong in chapter 1, but probably some Monastery=1 boolean somewhere fukt me. I reloaded after that to just finish as a Monastery boy because I figure the Monastery=1 boolean will cause a dead end at some point, and because there isn't any substitute nonfaction armor I could use like the various stuff in NotR.
Edit - Oh, and another thing hinting that I was gonna dead end somewhere: No one will open the gate so you can leave the monastery after the above process, so you gotta jump out again.