My FH works on cars all the time too, so that was a consideration of ours also. When we told the jeweler we were considering titanium, the first thing he said was that if my FH gets into an accident, they might have to amputate his finger because they can't cut off the ring. Great salesperson. We went with tungsten carbide instead. Here's his ring, but without that ugly diamond in the middle:
We got it from Jared and FH loves it because the grey squares in the middle are the same material they use as accenting on a lot of race cars and stuff. See if maybe your FH would like something like that.
Also, when a guy's hitting on me, I tell him I'm married and they still don't seem to care. I don't believe being "off the market" is as clearly defined as it used to be. I think your FH's "damsel in distress" girls are probably more interested in getting their cars fixed for free though. I wouldn't worry about that part of it as long as you trust him.
My fiance won't be wearing an engagement ring, and probably won't wear his wedding ring very often either. The reason for no engagement ring is because we live together- people know we're engaged, and he doesn't ever wear rings anyway.
We'll buy him a wedding ring once the wedding day gets closer, but we've both agreed that we're not going to go crazy and buy a really expensive one, because like some of the other posters, my FH works with his hands and with heavy equipment and machinery. I'm not concerned about him losing or damaging the ring, but him losing or damaging his HAND because he was wearing a ring and it got caught on something. His boss and cowokers are all married, and they leave their engagement rings at home. Some will wear it on a chain around their neck, but a lot of them feel uncomfortable doing that because a chain can get stuck in machinery just like a ring can. My mom says it's a good sign I trust him enough to walk around without a wedding band on :).
Also, I did a lot of research online about titanium rings, because my FH likes the way they look and the idea of having titanium jewelery. In all the things I've read, it says that while titanium is strong and not easily cut, if you go to the ER or to a jeweler, they CAN remove the ring for you. This seems to make sense to me: I don't know why anyone would sell or buy a ring that could cause you to lose a digit.