Thanks for your reply Matthew; we'd thought along the same lines that you've mentioned here. Our experience has indicated that the 8810 control outputs are, in fact, particular about the types of load they will drive. They appear to be able to source enough current to drive 18mA, however the voltage drops to <5 Volts.
Our application requires a 24V relay with a rated input coil impedance of 650 Ohms. We initially tried some off the shelf items from Radio Shack with the results I mentioned above.
Before we received your reply, we had already ordered solid-state relays from OMRON: G3NA-D210B-DC5-24, 7mA. They're installed and work a treat. They were not inexpensive but were a tiny fraction of the install cost.