Giving the potential for roof damage on any RV I would not carry anything on the roof... the resulting leak and eventual wall damage is enough to keep me from doing it. Maybe have 1 built to sit above the roof and not on it? Your kayak can't be that heavy, it wouldn't be that difficult, but I just wouldn't want it sliding around on my roof is all.
I would look for the roof rack from the blazer or a Bravada in the junk yards (same vehicle, just in chevy drag) and / or figure out a way tie it down on top of your olds. They aren't the difficult to install.
A Thule roof rack for 1 with no existing roof rack is kinda pricey (~$350), but giving the potential damage to the trailer, you could end up paying a LOT more later.
There is nothing preventing you from going down under the bumper and then tie to the frame... just put some foam or something to keep the rope from rubbing.
I can't recall ever seeing a Bravada without a roof rack... they usually came pretty loaded... (not saying you don't have 1, just don't remeber ever seeing 1 without).