1.8V and 2.8V LVCMOS I/O Support

I’ve been using the XEM6001 and now need to interface to a device with 1.8V and/or 2.8V LVCMOS I/O. It doesn’t look like the bank I/O supplies for the Spartan-6 are user-accessible.

Any recommendations? Is there a work-around? Is there another USB OK board with better support for custom I/O voltage?