There’s a few ways to do this; you already mentioned one of them, which is switching your mechanic to DSX for that group of notes, but you’d like to avoid that per your post.
The “purely USX” answer is probably to either plan your lines to switch after the upstroke, but if you can’t do that (as you mentioned in your hypothetical situation), then you’d do something like pull off the last note, that way you’d have time to switch to the next string.
Another way is to develop a “helper” motion for essentially DBX. The only one that I’m familiar with is finger movement, but I think there’s maybe some other ways. You can see some of my recent videos with what I think is USX with a helper motion for DBX.