Tell me I'm dreaming but maybe it could work.
I think you're right on the money with that "maybe", bazza00...
...but where would we be without dreams ?
The XDR1 receiver's output was set up to "replace" that from a typical guitar, in other words high impedance with (I would think) more than enough level to drive typical in-ear transducers.
When compared to a "genuine" in-ear beltpack, I'd expect the main sonic deficiencies to be in the bass region, which mightn't be a problem for your vocalist, given the amount of "thump" that usually kicks around on stage.
What will definitely be a problem is the X2's not "playing nicely" in the same "sandbox" as the younger Line 6 Kids. Their RF schemes are incompatible, which almost guarantees drop-out disasters. This, naturally, may also not be an issue for you, should everything else in your "dreamland" be cabled, rather than wireless.
To my mind, the XDR1's high level output may also work better being "buffered" before the transducers by "series wiring" their "conversion" to mono, should you decide to proceed.
This would probably easiest to do with a short adapter cable.
The easiest way to custom make one of those would be to chop the 3.5mm mini-TRS plug off a typical ear-bud extension lead, pare back and insulate its shield-like "common" (to the extent that it can't "short" to anything inside the guitar-jack plug you're adding) and then solder one of the transducers' inner "hot" conductors to the guitar-jack's "tip", with its "mate" (the "hot" to the other) then being soldered to the jack plug's "sleeve" (normally the "shield").
I'll be fascinated to hear how the experiment goes, if this reply hasn't proven too discouraging...