Splitting keyboard across multiple pads

I have a "Mame" box with two joystick pcbs with 2 sticks attached for a total of 4 joysticks.  I'm wondering if there is a way to configure rpcs3 like how Mame does it, such as just listening for inputs being pressed, but not tying a single pad to any one joystick id.  Or if it's possible to use the keyboard driver, but different keys for different pads.  That, combined with xpadder, is a perfect fall back for full functionality across the 4 joysticks.

Thank you...this is amazing work....can't wait to play 4 player Scott pilgrim on my arcade cabinet...and so glad to be able to play it without having to fire up my rusty old PS3.