Did you do a search for machinable ceramics? I used them decades ago for making bushings and supports for use at high temperatures.
I think the name machinable ceramic is a bit of a misnomer because they have to be fired after machining to bake them to the final ceramic form. The machinable form is a sort of pre-ceramic.
One thing you might want to consider is the possibility of out-gassing from a ceramic. Most ceramics have some porosity and I think the machinable ones are based on sintering and they may have even more porosity than normal ceramics.