I'm afraid not. Though, you could modify it via pan_setbutton! If you want to rescale the button bitmap(s) to an unknown scale, you could rescale the bitmap via bmap_blit and modify the button with pan_setbutton.
You can also set two panels instead of one, The first with just the background the second with the button you would like to scale but with no bmap other than the button and then scale that panel.