It can be done. I rock my 36", 28-pound son for all of his naps (my husband puts him to bed at night), and it's doable with my 30-week pregnant belly. I can't rock him for long periods of time, though, so if he wakes in the night and I know it's going to require a super-long time in the glider, my husband takes over. It's uncomfortable and puts too much pressure on my tummy.
I'm 29 weeks pg and lately DS2, who is 2 y/o, kindof drapes himself over my belly with his legs off to the side and his head on my chest when we rock. At some point he figured out how not to put too much pressure on my belly, probably because he had heard me say "gentle to mommy's belly" a million times. <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/wink1.gif" style="border:0px solid;" title="wink1">
I've nursed through two pregnancies and it was no problem, the kids figure out a way to reach the boob if they want it bad enough <img alt="" class="inlineimg" src="http://www.mothering.com/discussions/images/smilies/lol.gif" style="border:0px solid;" title="lol"> Seriously though, one thing that worked great for me was lying down whenever possible - nursing lying down was was very helpful.