You could use a jack with a break contact, or you could duplicate the trigger circuitry for that particular voice. For instance, for the snare drum you could add an extra 470K resistor joined to the junction of R30 and R28, and an extra diode joined to the junction of D3 and R32.
Separate outs may be tricky - you might have noticed that there are two "busses" that form the output, one passing through the accent circuit and one passing directly to the volume control. The BD output is easy - take another capacitor off the collector of Q1, and perhaps build a little buffer circuit for the socket. The RS output would come off the collector of Q6 and would be best with some sort of buffer too. Neither of these will have the accent applied to them - they'll be full volume all the time.
The snare and hihat outputs present a problem, because they feed an output through the accent bit and straight to the volume control, and in the case of the snare you've got both the "skin" oscillator and the noise burst. You'd need to work out the right proportions to mix them in, with a bit of experimentation.
I think I'll get some veroboard and start building a DR55 clone ;-)