Yeah, I've hacked the dreampad too, and the insides look totalluy different to the one on the website.
If yours is the same as mine there is like a ridge of presoldered pads,as you say, beneath the mem port which you can follow around the pcb to the related buttons, and use the c and z buttons as opposed to the L and R for the triggers.
As for the ground I actually soldered it onto the solder pad that leads to the black ground wire which goes back to the dreamcast.
It's a bit fiddley, but in relation to the sega pad I initially fried it's a doddle.