Wiring a Z-Wave 3-way auxiliary with neutral from other switch? (w/ diagrams!)
Practically speaking, this arrangement should work without an issue, as everything is on the same circuit and a smart-switch isn't going to draw enough current to significantly heat a metal box by induction.  However, there are some very sticky Code-compliance issues involved with funky neutral routing -- the 300.3/300.20 mess is not at all clear, and if you have metal boxes, what you describe is technically a Code violation unless you modify the box to put a slot between the switch-loop cable and the incoming-power cable.

A plastic box, though, is immune to these sorts of issues...but the neutral not being in the same cable as the hot could still be seen as a 300.3 type of issue by the AHJ.  So, you're on the right track, but double-check with your local electrical inspectors to make sure that they won't ding you on this!

Of course, running a 12/4 instead of the 12/3 for the switch loop lets you run a neutral with the loop -- simply connect the switch loop's neutral to the smart switch, and then do not connect the incoming-power neutral to the switch loop's neutral in the loop-end box, or else you'll definitely be violating 300.3(B)/310.10(H) due to the inadvertently looped/paralleled neutral!

And while you're at it with the walls open -- run a 12/4 for the other switch loop -- the NEC has a requirement for the neutral to be present now, even if you aren't putting in a "smart switch" straight away.  (Just wirenut it off at the unused end if you want to keep the dumb switch there)