I'm using the Seidio Active too and I'm a little confused by your statement. All the places you would normally grab are rubbers, plus there are indentations for your fingers...not sure how you will find anything more grippy than that?
Normally I would also suggest OtterBox, but it appears they do not make a case for the extended battery. And if they did, the Commuter case would be comparible to the Seidio Active, though OtterBox uses a harder and more slippery plastic shell and the same type of rubber insert, so I don't think it would help you.
So, I am sort of at a loss here other than maybe getting some bedliner and spraying it on the plastic. Don't know if it will adhere to it though...