The whole reason for avoiding mixing isn't about conflicting between the technologies; it's about the fact that the M1 has 256 lighting addresses going back to the X10 days and UPB and Z-Wave are overlayed over those spots via some translation. With 2 different technologies it's easier to end up with multiple devices landing on the same Elk Lighting address. It's very easy to renumber UPB devices, so set up the Z-Wave first then move the UPB devices out of the way.
The Elk will monitor events on the lighting side; you can trigger the elk to Arm based on seeing a link id if you really wanted to - or turn on a sprinkler, or just about anything else. It has trouble keeping updated status of switches that were activated via a link
Per the original OP's question, yes - the Elk can be set to receive a string via a serial port and do something - it can't parse the string or do anything dynamic, but you can set a rule that if it receives "xxxxxxxxxx" on Com2 then [execute rule/task]. And you can use dry contacts - any zone can be set to do just about anything upon the contacts opening or closing; either via rules or even a couple built-in functions (like keyswitch).