Unless someone else has a recent report of trying this, I'd say the only way to know for sure is to go through the online process as if you were going to buy miles, and then see if Travel Bank is available on the screen as a payment option. If it's not, then you know the purchase is not eligible. The T&Cs say that TB is for the purchase of air travel "and as otherwise permitted by United," so they have given themselves a lot of wiggle room.