I can't think of how to add ventilation to rain pants and still have rain pants. When I bike to work in the cold, I just use breathable nylon pants that get wet but stay warm.
The problem with removing the crotch/inside is that you will get water spray from the wheels. The problem with removing from the outside is you will get rain spray. The problem with chaps is your butt is one if the wettest part of you after biking in the rain. The problem with rain pants in general when biking, is that your legs sweat a lot (but because they are moving they dry off fast). This is biased on my personal choice in cold wet clothing but, I would go with chaps worn over supplex nylon pants or the inside seam, with breathable front and full fenders. If you do chaps, make them real chaps that can be removed quickly.
Dont forget booties, probably a better way of staying comfortable in the rain. And fenders.
Sorry I could not be real helpfull, rain pants aren't my thing biking or hiking.
Scott