Hong Kong currently only requires one-week quarantine for qualified travelers, but it is still an isolated and expensive requrement.
I beleive transit is possible at HKG, but they are so limited flights to/from Southeast Asia, so it is not just an UA issue. It is an HK issue,
Now UA and SQ has a full code-share agreement, suggest OP to look at flying to SIN via SFO on UA and conenct to other Southeast Asia destinations via SQ,,,much better airlines and nicer airport. The new Silverkris and Krisflyer Lounges are very nice at T3.