I supposed the flail would be hydraulically powered? If so, I suspect the hydraulic flow rate on our smaller units may be a consideration to running one?
There is an on going thread about a lane shark which is sort of a brush hog mounted on the FEL. Right now the OP is trying to get it set up to run and the hydraulic plumbing seems to be thought provoking.
I’d sort of like a FEL mounted post hole auger and I know only specific units work on the low flow hydraulic systems most of the Ls and smaller have.
Sometimes I’ll mow in reverse with my L and the brush hog when I’m in really over grown stuff. That accomplishes what you mentioned. As long as it isn’t hours at a time. Be warned, when you tear the tail wheel and bracket off and set the BH on it, things get loud and expensive. My dealer told me you keep mowing in reverse and I’ll keep selling you parts.