I decided to simply create directories within /mnt, chmod 000 them and use them as fixed mountpoints;
for manual temporary mounts I have /mnt/a, /mnt/b, … /mnt/f, but I never needed to use more than two of them at once.
While this setup doesn’t really respect the filesystem hierarchy, I wouldn’t have used /mnt at all if I were constrained by its standard purpose since having one available manual mountpoint seems pretty limiting to me.
Then again, I have 3 physical drives with ~ 10 partitions, plus one removable drive with its own dedicated mountpoint…