Does anyone have suggestions as to third-party tools that could be used to author `.mtlx` files for import into Houdini?

Substance Designer has a half-baked plugin for the 2020 version but it hasn't been updated so I guess Adobe don't think MaterialX is important and it looks like Quixel have nothing at all...

Does anyone have any other ideas?

I haven't seen a tool yet, but AMD is sponsoring a libray.
https://matlib.gpuopen.com/main/materials/all [matlib.gpuopen.com]

Yeah I've seen that library; it's very good. There is no clue as to how they authored the materials though.

I am thinking it be possible to use Quixel Mixer to generate the texture and then python to put the mtlx file together (possibly using the python binding from the official MaterialX library [github.com]...). That would mean no custom shader nodes, but might be good enough for now...

You could keep an eye on Oliver's OdTools. I believe it can cross convert materials between render system.
https://origamidigital.com/cart/index.php?route=product/product&product_id=66 [origamidigital.com]

Yeah fingers crossed it will support MaterialX shortly...