i think part of the problem is that currently the play key could control other programs (such as vlc if it is running). having the keyboard media button start banshee would break that behavior.
i recommend using gnome-do. then you can just summon do (ctrl+space), start banshee (start typing banshee, hit enter when gnome-do figures it out), and press play on the keyboard. note that i've remapped gnome-do to ctrl+space since the super key summons unity's search function. the default is super+space.
this solution is probably more keystrokes than you want, but it is pretty fast if you are accustomed to using gnome-do (which is awesome and you should look into anyway!). there is a banshee extension for gnome-do, but it is currently broken (in ubuntu 11.10, anyway). hopefully it will get fixed soon, and you'll have an even quicker way to get your music playing.