The official plugin for mekanism only adds recipe handlers for the metallurgic infuser that use tags. Since there are no tags created for custom infusion types (and gases), they can't be used to create recipes. I have not tested whether custom gases are usable, but I believe the recipe handlers aren't set up to use tags and would therefore work.

I suppose I should've specified that it was the official Mekanism plugin for KubeJS, made by the KubeJS dev. It only covers recipes, and even then not all recipe types.

What is "the official plugin for mekanism" ?

Metallurgic Infuser expects a 'tag' for its infuse_type input. The infuse_type created by this mod is untagged and afaik its not possible to assign a tag to it, so there is simple no way to use custom infuse_types anywhere.