How to backup seed in lightning and give your channel to watchtower
For clightning I have created a video that explains how to restore your node with seed and state you can watch it here: https://youtu.be/-S7pQOw36vI

The general idea is that hsm_secret sites your seed in binary and the sqlite3 datebase sites your channel states. 

I believe in lnd the process would be similar only that they give you the seed words explicitly instead of hsm_secret. They should also have some database or file in which they site channel state. 

As for watchtowers. Currently to the best of my knowledge there are no such services available.