OK, I'll check out a trial version of Lightroom.
ACDSee also uses it's own database. But it allows you to embed database info into image file - it adds this info to XMP, either embedded in file or as sidecar. With JPEG it seems to always be embedded. So "rating" is not any standard IPTC field that I know of, but since it is put into XMP, I have hope that I will be able to somehow get this value into the Lightroom equivalent.