gamecat-
If it were my campaign, I'd let them do it as an "off-hand" attack . . . but of course they'd at least three feats to use it effectively, or a level of monk and two feats (Ambidexterity and Two Weapon Fighting). Even then, they'd get next to no damage from the kick. Why not? They'd be penalizing all their attacks by -2 for an extra d3+2 or maybe d6+3 damage, at most.
The only house-rule part of this exercise, if it were my campaign, would be to rule that you only get half your strength bonus to damage with the kick, because most of your body's strength and leverage is spent in whirling that heavy blade all over the place.
Of course, you could always say "Sure, you can kick this round and swing your sword, but you'll have to use one of your iterative attacks to do it".
-S