Love my 556xi but hate the handguard. Is there any way to retrofit either a classic sig habdguard, or a quad rail, into that spot?

Not that I know of. For all its talk about “modularity”, the xi was really a dumpster fire. All I know of are the two factory options by Sig; the metal handguard with screw holes for the proprietary mounting rails, and the polymer vented handguard.

The problem is that the xi uses a handguard that’s free floating and slides onto the monolithic upper of gun, similar to the MPX/ MCX, whereas the front of the Classic Sig handguard mounts into the gas block, similar to a classic AR15.