Of course BS.Player supports change of aspect ratio
(e.g.: [Shift]/[1]: original; [Shift]/[2]: 16:9 and [Shift]/[3]: 4:3)
and in fact you can also add your own defined "Custom aspect ratios",
but I don't think (unless I misunderstand your question) BS.Player presently offers an option to choose between display aspect ratio or pixel aspect ratio.
Therefore moved your post to "Feature request and suggestions"-forum. 
I suppose you do know that BS.Player in fact supports
any aspect ratio for playback (and e.g. that in window mode you can freely resize window)?