I'll allow it, because:
1) An attack is a standard action/Cleave says as a standard action perform.
2) Cleave isn't a full attack, it's a single attack, with an effect to roll an additional, not new, attack. You aren't taking two attacks (RaW) you resolving an effect of the first.
3) It makes MORE sense (Thematic, RaW, IC) to permit Cleave FOR someone mounted.
