Hi Simon,
You currently can't do this because Pixy uses a bootstrap mode for loading code and updating the flash. The bootstrap mode can only be entered by pressing the button --- it's an NXP thing.
We could add flashing to the main firmware in the future (so we don't need to enter bootstrap mode)-- that might be cool.
In general, JTAG is probably the best way to develop code on Pixy. We've had some problems with JTAG in LPCXpresso though--- problems with flashing! so there's been a bit of a rabbit hole. Hopefully these issues will be sorted out....
thanks,
--rich