From the FAQ section on Gerbing's web site:
"Q: My bike already has a pigtail for my battery charger/maintainer. Can I use that to power Gerbing's Heated Clothing?
A: Unfortunately, no. We understand the desire not to have any more pigtail leads poking out from under the seat than is absolutely necessary. But depending on the device, some of these battery charger leads are only capable of handling the 1-to-2 amps that such chargers deliver. Using them to power from 4 to 15 amps of heated gear can blow the fuse and, even with the proper fuse in place, can overload the capacity of these same wire leads."
Now this speaks to using the harness from the Battery Tender to power heated gear, not sure how it would operate in reverse (using the heated gear pigtail to run the tender). The newer Gerbings gear uses a coax style plug that doesn't match up to the Battery Tender plug, although I've seen adapters that allow them to connect.