Is it possible to do non-uniform scaling on height fields (in X or Z, as opposed to in Y). These options don't seem to be available in the HeightField Transform node. My workaround has been to convert from HF to mesh, and then back again, but it gets a bit clunky when working with dense height fields.

How do I transfer float attributes from a mesh to a height field? The HeightField Project node seems to discard any additional attributes when doing the projection. The docs [www.sidefx.com] only seem to mention transferring height data, but not other attributes. Perhaps another node is better suited?

Heightfields are just 2d volumes. You can transfer any data using the volumesample vop.
Scaling in x and z is a bit more tricky, but it can be done using the volumesample node as well.