Instead of letting the Ledger choose a random 24 word seed and generate addresses from it, I wonder if it would be possible to generate a vanity address (probably offline) and import it in the Nano S.
Please stay constructive. If you think it's impossible, back it up with an explanation ;-)
If you do any of those operations, in a way of another, you weakened the security of your wallet, at least in theory. The purpose of the hardware wallet is to do everything by itself, exactly to obtain as much security as possible.
Importing one private key or a list of private keys is not possible.
Now, for the purpose of experiment, let's say you find a seed that creates a vanity address you'd like. You know that the purpose of those seeds is to generate all your addresses from the wallet from that seed, right? And then only one of those addresses will be your vanity address, which, for safety purposes, should not reuse after spending from it.
So, with a great effort, you may get one vanity address, which you probably will not use for too long. I don't see it like something that worth it.
Could a generator generate keys and seeds?
As I explained, it would be a rather useless effort.