I tried manually setting .mrepro to 50 using the console, on a bot with 13k energy, and nothing happened. The next cycle, to make sure the bot wasn't unable to reproduce, I tried setting .repro to 50, and it reproduced properly.
Looking into this in the source, it looks like although mrepro does trigger reproduction, the routine which processes the reproduction reads from only .repro to get the percent-of-energy parameter. Since I'm only setting mrepro, it reads 0 from repro, which is probably why nothing appears to happen.
(I'm trying to have a bot set mrepro on another bot through a tie, so setting both mrepro and repro in one cycle isn't possible)