but I don't see any reason to assume that you have to be specialized for arboreality in order to be an effective glider.
No, but if you're not already arboreal, what point is there in gliding?
This is not true for all arboreal gliders. Some gliding amphibians have flaps on the distal limbs, such as "flying frogs" (_Rhacophorus_ spp.) which have huge webbed feet for gliding.
The "flying geckoes" (_Ptychozoon_ spp.) have gliding surfaces on the abdomen and feet that are not confluent.