I know this has been covered but I "discovered" something I think good for back pocket in case of travel etc and can't get to rings or bar for awhile.
While it is true that ab wheel roll outs will improve from front lever training alone but not much the reverse I think there is a way to tweak the ab wheel work to give FL benefit.
Does this make sense?
Instead of doing standing roll outs, do them from and back into plank position. This means using hip flexors only for stabilization and all the dynamic work is done by arms, chest, lats, etc. in same plane and range as for FL. Even more so if using the two wheels on an axle for each arm set up type ab wheel.