These evil maids are even more implausible than that. Has to be ridiculously targeted. If you are really targeted by such a powerful state-like entity, wouldn't it make much more sense for them to just send a NSA letter to Intel (or whatever the weakest link in your chain is, and there are plenty of extremely weak chains here, like the BIOS manufacturer) and/or backdoor the hell out of it?
Secure Boot was never about security for normal users nor security for the majority of us. This is like https://xkcd.com/1200/ all over again. At the point the attacker can write arbitrary bytes to your hard disk, its way past the point where the majority of users care.
But yes if the OS also let’s you change the boot ROM then you can make your root access semi-permanent.