Glassrom has signature spoofing and allows microg to install as long as it is signed by the ROM's release key
In simpler words you need to build it yourself, sign with your own keys and then sideload the microg apk signed with your release key
Since glassrom tries to provide comparable security to graphene on devices where porting graphene with everything working is hard or even impossible there are no public builds of glassrom for devices officially supported by graphene
The signature spoofing in glassrom is not meant for microg. It's an anti-tivoization measure I'm experimenting with and its main purpose never was to run microg