Have you tried sleeping in the bag quilt style - i.e. zip up the footbox to put your feet in, open the bag out flat and sleep underneath it - no bag underneath you just your sleeping pad.

I'm a big sweaty guy and I have the same problem - it's usually much better when I do the above, which is probably 95% of the time I'm in a sleeping bag.

The other thing I find *really* helps me is to bathe that area at the end of the day - even if it's just water - which it usually is for me. I take a dip, or even a little "sitz bath" type washoff using a pot or two of water - YMMV.

I've never tried the powder..