Jeez Ed, I reckon you could do whatever you want with your own figures. It's not as if the bust police are gonna kick your door down, haul you away and try you for crimes against perfectly good full figures.
If it's exhibition/competition rules on busts you're concerned about I reckon the piece you describe would be considered a bust. As we all know the original idea of a bust was mainly a head with some chest and possibily some shoulders but mostly a representation of a head and a recognizable face.
But these days busts which are currently commercially available as kits for painting can include the whole upper body with arms, so I can't see how your figure which would represent the same type of thing wouldn't be considered a bust.