Cross-chaining on a 3x6 system -- how bad for wear? I keep an old 3x6-geared MTBSO near work, and the front shifter is currently broken (smashed plastic gripshift).  It will be a few weeks before I can get it home to replace it with a spare, so I've currently used the FD limit screws to fix it into the middle chainring.  This leaves the gearing a bit low -- I don't have much accleration once I'm spinning over 100rpm which makes keeping up with traffic hard -- so I'd like to fix it in the big ring, but then for pulling away I'd be geared big-big.  

Is this likely to significantly increase wear on the gears themselves?  It's only a BSO so I wouldn't mind, except I don't think the sprockets are ever coming off, and if they did, replacements would cost about as much as the bike is worth.
I would not be concerned in the short term, say 6 months to a year. There are several things in your favor. A six speed chain is wider than a modern 10 speed chain. Wider equals more material equals longer wear. On a MTBSO parts are used that are cheap. Cheap means steel instead of lighter alloys. So I think you will see some wear but it will be minimum.