It won’t be much help for routing purposes. Connections have to be routed from very first packet and there’s nothing identifiable in SYN packets.
Basically there are two options:
a) Find networks used by Google and route them as whole, but it will include more than just Youtube.
b) Do some trick with address list, where you can add addresses found by L7. Then mark routing based on this list. It won’t work for first connection, only subsequent ones.