I second Delta 100. Its delicate highlight scale is excellent for what you're doing. I would, however, suggest developing it in PMK. Highlight separation is Delta 100's great strength (along with startling sharpness) and it combines superbly with PMK. PMK's staining property seems to extract the maximum in highlight detail and sharpness) from Delta 100.(posted 8462 days ago)Xtol, from my experience, tends to block highlights. I will concede, however, I have not used it for some time, and Kodak had some problems with shelf-life, which, from the posts I've seen, seems to have been corrected in recent production runs. So it's possible the sample I tried was not representative.
Nevertheless, in terms of grain, sharpness, smoothness of tonality and highlight detail, I cannot imagine Xtol besting PMK.
One more thought: Fuji's new film, Acros, also has wonderful highlight separation. I've only shot a few rolls of it, but from what I've seen, I've been very impressed. It's much like TMAX100, but with better highlight detail. It has extremely fine grain, with a grain pattern that is micro-smooth--very similar to TMX. That nearly grainless, almost diffused look, coupled with its lovely highlights, might make Acros an even better for the dreamy look you want. Delta 100 is razor sharp and could be too sharp-looking to suggest dreaminess.