There is a workaround:

First, find an non-BE, UA-metal itinerary that is at least as much as the difference between the ETC and desired itinerary (e.g., you have a $500 ETC and want to buy a $800 ticket, find an itinerary that costs at least $300 - it may take a bit of searching to find the one for the right amount, but Google Flights and the price sort function on united.com are helpful here). Book that itinerary using TravelBank.

Then, call to change that itinerary to the desired itinerary, paying the difference using ETC. United.com will only let you pay by credit card, but the phone agent can do it, as long as the ETC (one or multiple) covers the entire price difference; you cannot pay with a combination of ETC and credit card.