Found something in Hungarian, short Abstract as intro in English.
http://www.rmki.kfki.hu/~lukacs/RUSYNS.htm
Seems to be a presentation on a conference, some historical stuff, but given by a physicist. Chiefly about Northern Hungarians and Rusyns, but a section is about White Croats.
The author also knows about that document mentioning ~100 000 White Croats (he gives a link where it is/was published, it seems it redirects to elsewhere now), emigrated to the US from around Krakow. He adds, that up to about 1860 the average Polish serf did not call himself a Pole, those were the nobles, but he identified himself as a local ethnicity. He does not mention how this Bielochrovat ethnicity could survive about a millennia without being mentioned anywhere, tho it isn't his point anyway.
In the Appendix some further info. About the founders of Kiev funny coincidence - elsewhere this was our topic, that they were Alans, and the Alans supposedly had three tribes: Serboi, Choroates, and Antae. Choroates seems to be the Croats. The Nestor chronicle preserved three names of three leader of the founders Kyj, Scsek, and Khoryv - in the third Croat was preserved. As elsewhere we talked about, a theory identifies them as Hungarians: Kő, Csák, and Geréb.
Croats being Alans originally would mean, they had Iranian language first, and they changed to Slavic later during their ethnogenesis (similarly how Bulgarians left behind their Turkic language).
Hungarian Wikipee also calls them - what I could transliterate as - visyan or vislan.