@labris, I know there was support put into our 3ds Max exporter to handle custom attributes written as extras in the glTF file. There isn’t support in Maya for custom attributes when exporting to glTF because, as I remember, there wasn’t a great way to set these custom attributes in a way we could access them for export. Beyond the DCC tools that we had created exporters for, I don’t know of others.
I know that Houdini has glTF import and export and I believe that Modo does as well. However, I don’t know if either of those handle custom attributes. Substance painter can also export a glTF, but that one definitely does not handle custom attributes.