How can I access a bitcoind instance from another device with wallet functionality?
I have seen selfmade PHP frontends. May check google, forums, github or other sources. (want to avoid to post links there).

To answer the question properly: You can access the bitcoind instance through the RPC and do all kind of things. This indeed allows to use existing or selfmade frontends.

For reference on bitocin RPC check the popular forums or google up "bitcoin rpc commands"

If you intend a connection between a private server and you home server you could secure the copnnection additionally with a VPN for example.