The problem here is that OpenWrt expects /var to be a symlink to a tmpfs backed /tmp/, changing this to a permanent location will break some deeply ingrained expectations around this assumption - and you will see both 'spectacular' and subtile breakage from changing this.
Food for thoughts (just scratching the tip of the ice berg):
- /var/ usually is on tmpfs and tmpfs disappears while rebooting, so cleaning up stale files isn't necessary
- /var/run/
- /var/lock/
- /var/tmp/
- /var/ can't be on the overlayed rootfs, as applications have a reasonable expectation of inotify working on /var/ (overlayfs doesn't support inotify)
- there are undoubtedly some applications accessing /tmp/var/ directly (or indirectly, by traversing the filesystem)
- early boot issues (/var/run/ must be available (in writable condition) pretty early on in the boot process)
- over-mounting issues (locks and states may be created early during the boot process, before the final fs is mounted on /var/, shadowing already existing files that are still in use)
- …
None of these are unfixable (but fixing these will need more space for cleanup scripts) and they probably should be fixed, but the early adopters and proponents of these changes (as in you) will feel the pain of finding the problems and coming up with solutions. Especially as this will 'always' remain an expert's setting (on most devices you'd both run into problem with flash size and -wear), so less eyes to find and fix issues..