There's actually a fairly simply yet eloquent solution...
Keep heavy. But make it only apply to melee attacks or spells that already specifically gave it as a status ailment. As a trade off give cast spells a % chance of knocking you off your mount. The % chance decreases based on your MDef and level difference, thus making it nearly impossible for a lvl 41 sylph to knock a lvl 50 blm off their mount, but a lvl 41 would still be legitimately threatened.
The major problem most players have with Heavy is that it applies from cast spells, which can hit you even after the mob has leashed. Getting in melee range while on a mount can be avoided if you're creative enough, but spells...nope.