Datapoint strongly suggesting (although not definitively confirming) that TravelBank funds CANNOT be put towards the purchase Economy Plus subscriptions:

- Yesterday I tried to purchase a $999 Economy Plus subscription (global for myself and one companion) using $200 of TravelBank credits, with the remaining $799 going on my Amex card. I got a $799 pending charge on my Amex but my TravelBank balance stayed at $200, and I neither received a confirmation email nor did the subscription show up in my MileagePlus account. 24hr later I called United to see what the problem was, and the agent told me that the purchase "didn't go through" but wasn't able to provide any more details.
- So I tried to purchase the subscription again just now, but this time not using any TravelBank funds, and it immediately went through successfully (membership is active, confirmation email immediately received).