ribs are simply scene description, as long as you comply to the rispecs and make a rib file you can render stuff in renderman.
Using mel is going to be really slow. But if you want to take that route, then there are mel commands to todo file I/O and simply convert the meshes to the rib format and you fire off a command line and render that rib… I frankly would use maya api, it will be superfast and you can do more…
There are rib generators out there for maya, liquid maya is one free one out there http://www.highend3d.com/downloads/tools/3d_converters/3680.html
MTOR is awesome, but comes at a huge price…