Given warp-field technology in Star Trek canon, is it possible for a non-warp ship to use the warp bubble of another (warp capable) ship to exceed light speed? A sort of nascar 'drafting' if you will.
Yes, you can 'ride' in the wake of a warp capable ship, despite not having a fully functional warp engine of your own.
In the Enterprise episode "Unexpected", we see a (cloaked) ship hiding within the Enterprise' warp field. At the start of the episode, they're both travelling at warp. After their discovery, the following exchange occurs;
TRENA'L [OC]: I apologize for any damage we've caused. Our engines are malfunctioning and we've been using your plasma exhaust to replenish our teraphasic coils. It's allowed us to share your warp field.
T'POL: Their warp reactor is offline.
ARCHER: We can't help you unless we can see you, so why don't you disconnect whatever stealth device you're using. (the ship appears aft of Enterprise)
In ENT Divergence we see Tucker being transferred at warp from one spaceship (Columbia) to another (Enterprise). This is made possible by the two ships flying so close to another that their warp bubbles touch and Tucker never leaves a bubble. Assuming that Tucker has no warp-sustainer device and is otherwise incapable of travelling at warp speed it is to be assumed that for him the same laws apply as they would to a ship: They can sponge off a warp bubble and exceed light speed.
When a starship tows another starship, it is able to bring a ship a long with it at warp speed using a tractor beam.
To safely tow a vessel at warp speed, the target vessel's engines had to be deactivated to avoid shearing forces against towing vessel. http://en.memory-alpha.org/wiki/Tractor_beam
Presumably a small ship inside the warp field of a larger ship moving at the same speed and direction of the warp ship, should be able to "draft" the other ship's warp speed.