It's easy to calculate warping on-the-fly. Just define a value of, eg. 0.25 to add to a cumulative scroll variable each scanline. And increase this value per frame, presto, the background warps. (You can see it in the intro to Tongueman's logic when my name comes up.)
Your scroll variable will have to be stored as 3 bytes -- 2 bytes for the usual scroll reg and a 3rd for a fractional value. (0.25 would be stored as $40,$00,$00 as the 6280 sees it.)
