If you have a crown made from a ceramic which is 99.9% alumina (well
the core part anyway, i guess the rest would be built up on top to give
better anesthetics),

my understanding is you cant bond to alumina, only silica to tooth
tissue. These crowns eg Allceram dont have silica on their fitting
surface, cos they are 99% aluimina.

Would you use RMGIC/GIC to cement them, or am i missing something?

In ceramics, there is something called glass ceramics which are better
suited for bonding to tooth tissue ie veneers and dentine bonded crowns
are made from these.

Can you use PJCs, or other crowns which have an alumina core content of
between 50-80% to become dentine bonded?

Thanks