I prepare to install OpenVPN server in hAP ac2.
It is possible but it will just be a server with very limited options.
After all this I start to think it would be better when MikroTik simply relabled the OpenVPN feature: name it something like MikroTikVPN and don't suggest any compatability to OpenVPN.
Then prospective buyers will no longer be deceived into thinking that they can use this OpenVPN feature to interconnect with some other OpenVPN service or client.
It would make it just an incompatible variant of SSTP and it could just as well be dropped entirely, but keeping it so existing users do not have to rework their setup would be nice.
As an OpenVPN server or client, well, it is just worthless.