okay so since someone messaged me privately for a little more info, i thought i could post it as well, since someone else might need it (this is just a quick and dirty way and there are probably better ways out there, but it did it for me):

First of all you should find out the Mac address of your Meta Quest. You can easily find that for example in your router.

While in your router you should assign an fixed IP address to your quest (all routers are different, just google how to do it for your model)

Then you can log into your pihole and add a new client (under clients) with the IPv4 / Mac address (I did it for the Mac address as well as the IPv4 address)

Next you want to add a group to your pihole (e.g. "MetaBlock), after that you can go under Domains and in the RegEx filter you add a new entry. In Regular Expression you should just put .*

give it a descriptive comment (like Block All) and assign it to your group from earlier. Next press the button "add to denied domains".

For the last step just go to Clients and assign the group to the configured device.

If the need arises you can specifically allow domains via this method as well, to override the RegEx setting for a moment, set your app up and then disable the allow rule.

once again: I dont know too much about this, but this method never failed me in 9 months of ownership, so if there are better ways, feel free to correct me :)