Custom G28 command

Hi. I want to take full advantage of some slic3r features, one of them the ability to use a custom bed shape. For that I need some custom homing routine. I can do it inside slic3r since it lets me customice the start Gcode to be use at the beginning of the print. But it daesnt work if I use the plate contro, and issue a "home all" command since it uses the G28 command from the firmware.

So my question is: is it posible to mdify the G28 command inside the firmware so every time I invoque it it executes my custom homeing routine?

Thanks in advace