Can you easily and cost-effectively run one-color jobs on an AB Dick Century 3500-S, or does it put wear and tear on the second print unit?

If you haven't figured it out yet, I run a 3500 and run the colour on tower 2 with roller lube on tower one. If it is 2 sided with powder then I put the roller lube on tower 1 leave the lock out pins in and the water metering roller off and without a plate ( like in washup mode ) back off the plate cylinder. Run tower 1 like you are printing over all varnish. On my press there is enough clearance between the plate cylinder and the blanket so the roller lube won't transfer to the blanket, as well as while in impression will take the powder and crap off the sheet with less powder build up on tower 2. this has worked well for me.

You can run the colour on tower 1 reversing the towers above.