Much appreciation to the information in this thread, though it took a lot of digging. I had some very old pre-pandemic ETCs about to expire, but didn't add up to a huge amount, about $125. So I wanted to book with those to finally burn them off fully and pay for the rest with my travel bank funds. Well low and behold, you can't do it, after applying the ETCs it would only allow me to pay the balance from a credit card, absolutely no Travel Bank option. Thanks to the wisdom I found in this thread, I found that if I booked 2 tickets (one with the ETCs and one with the Travel Bank funds) and cancel them after 24 hours then they turn into future flight credit, which low and behold you can use multiples of for a single new ticket.

God bless Google flights to find non-BE flights that were within $2 of my target price for both the ETCs and Travel Bank. Definitely not a straight forward process most people would come up with on their own, a great thing about the breadth of knowledge on Flyertalk.