The ASUS ZenScreen MB16QHG is a portable monitor w/WQXGA (2560 x 1600) - don't have one yet but considering it. This is one of their best portable monitors because it has a fold-down base with inputs (no wires sticking out of the sides of the display) and a full-size HDMI port unlike a lot of their other ones. I have a similar one in 1080p and it's great. They don't unfortunately make a 1440p portable atm (some no-name brands do).
I'm pretty sure the answer is no but is there a chance the MiSTer would support this resolution? afaik the highest resolution the MiSTer supports currently is 2048x1536@60 (default #13). If the mister could feed 2048x1536 with additional black borders to make the output canvas 2560x1600 that would be fine too but assume that wouldn't work either.
I could have the MiSTer feed the display 1/2 res (1280x800) but the 800 doesn't scale anything very well in integer, and the display's own 2x upscaling might suck (or might not). And I wouldn't get the benefit of thin scanlines and other crt looks, it would be worse than just using a 1080p monitor.
(I'm sure the RetroTINK 4K can do 2560x1600 and anything else with the custom modelines but I don't have one and that would be one more box to add to what should be a portable setup).