They do underquilts OK. Not ideal, but you can make them work.
I attach the underquilt on our Brazilians in an aysm manner. I attach the head-end to the far right cords of the suspension and the foot-end to the far left cords (or the other way around depending on how you position yourself). I tend to sleep on a pretty hard diagonal in these hammocks, so I do occasionally feel the edge of the UQ 'slip' past my foot. That can be solved with something as simple as a binder clip attached to the edge of the hammock and the hem of the UQ, to prevent it from slipping.
I used normal underquilts on our Brazilians (and other home hammocks) for years before making ones that fit properly out of el cheapo walmart supplies. Doesn't need to be lightweight when it never leaves the bedroom.