Hmmmn - it should work - from my page:
OTOH, the I-PAC supports key repeat on the 1 Up and 1 Down inputs only in PS/2 Mode. In USB mode, key repeat is supported on all inputs. Andy Warne can supply an I-PAC supporting key repeat on all keys in PS/2 mode, if requested.
So as long as your joystick is wired to 1Up and 1 Down, those should scroll.
You aren't using any of the HotRod or X-Arcade controller options with EmuLoader are you? (but then it wouldn't scroll with a keyboard).