I didnt found one yet. But i just started using Houdini, so perhaps one of the official houdini people can post a link.
I've also tried the unreal_instance attribute as mentioned here
https://www.sidefx.com/forum/topic/46689/ [
sidefx.com]
got the same bug that the assets where invisible during gameplay. but, when baking out everything works and was instanced correctly.
Concerning the substances I think both methods could make sense.
When asigning the substances in houdini as you mentioned, i think the substance parameters could get controlled from the interface, so each placed asset (in unreal)could have its own texture set.
Advantage of using the substances in unreal is, that they could get reused instanced and manipulated for every other unreal asset as well which might safe some performance.
The nonplusultra might be a combination of both methods, using the substances in unreal, editing the materials to use dirtlayers or any other overlays and generate dirt and alphamaps in houdini.