Would this work? Morley MHE 2-channel Stereo Hum Eliminator

EDIT: They're proving difficult to find. If someone was committed to avoid putting an output module in their main case, I suppose they could bring along a tiny separate case with an isolated output module and maybe a few other utilities.
-- CardiacTasty

Directly from Sweetwater's listing for this:

"Note: The Morley Hum Eliminator isn’t intended for use with mic- and instrument-level signals or powered speaker lines."

So, no, it won't work unless you stepped the levels down to line-level before hitting it. Given that, I'd say just go with a stereo balanced-out module in the cab, because a stereo attenuator module would take up about the same space.