It's definitely possible to make a ramp out of an FOF. I've done so in my Sonic 1 Level pack (Made from a few years ago), which wasn't really a level pack, more like a remake of GHZ1.
Yes, it's tedious and challenging, but definitely possible.
EDIT: I misread your post a little bit.
Surround the ramp sectors with an invisible and intangible, FOF. Make the Invisible, Intangible FOF larger than the numerous ramp control sectors (that you have outside your level) and have the sectors, in your level, tagged to the Ramp FOFs that are produced from the control sectors outside the level AS WELL AS the Invisible, Intangible FOF.