Fighting from a Handstand?

[Varyon]

I'd be strongly tempted to keep it simple. A handstand-kick is a decent example of an Acrobatic Attack (indeed, possibly one that uses the Acrobatic Kicks Perk, using Acrobatics in place of Brawling/Karate). Doing handstands and cartwheels and the like may well be appropriate for an Acrobatic Dodge (or Leg Parry). Of course, doing a handstand rather limits your mobility, so you probably can't Move more than a Step, although this largely avoids the benefit of an Acrobatic Attack... but someone on handstands doing kicks isn't really something most people have prepared for, so the kicks might be harder to avoid.

With all that in mind, I'd be tempted to simply allow the character to take a Perk, call it Handstand Combat or similar. This allows you to switch to a handstand as a free action (or maybe Change Posture, adjust to taste) on your turn. While in a handstand, you need to make an Acrobatics check each round. On any Success, all Deceptive Attack kicks impose a further -1 to the enemy's defense, kicks no longer require a roll to avoid falling on a miss, and all defenses are treated as Acrobatic (for +2). On a Failure, you fall prone, must make an HT roll to avoid Stunning, and all of your defenses are treated as you having attempted and failed an Acrobatic defense (for -2). On a Critical Failure (and, optionally, any Failure by 5+), you also take damage as though you fell from 1/2 your height (and, optionally, the roll to avoid Stunning is at -5).