i guess it could alternate between (a) standing soldier and (b) kneeling soldier
same hues, same res. A slightly less complicated option is simply draw a highlight of some sort around the button, perhaps the appearance of it depressed yeh.
heck, i'd be happy with hot pink "KNEELING" flashing up in the corner when the soldier is selected ( uh no, not really )
Edit: I just noticed there's already a standing figure for inventory. So, an alternate to depressing the button could be to just ghost it blank.
(or change the inventory icon to a rifle or something..)
/anyway, am thinking too much about this.