Your scheme should work well. As macrackit says, I would use a transistor at the master's tx, with a pull-up to +12V. Then fan out the output to all the slaves.

Did something similar, but it was the other way. All slaves *sending* to a single pin of the master pic. In this case, I had to use a rudimentary scheme to cater for the collisions.

Regards,

Anand