If you've got a fine drill, drill a 1-2mm deep hole in the bum of the seated figure, tiny dab of superglue in that hole, then poke in a wire spear or other piece of stiff piano wire. Then sink the other end of the wire into a cork - the type of corks with plastic tops attached like you get on port and sherry bottles are ideal.
Hey presto, you have a handle to hold the figure while you paint it at your leisure.
This is how I always do riders, seated and lying figures...