Hmm,

Could it be to do with this FAQ entry? Which lines do the ariel, headlight sense, amp and mute use?

If this is the case, does ttyS0 or ttyS2 have a DTR line burried inside the Empeg somewhere (I think not), or can I tickle some other pin somewhere in the Empeg that would (or could) cause the Empeg to fire a software interrupt every time the line status changes?