A commenter has discovered two manuscripts of the Old Slavonic Methodius online! The manuscripts used by Michael Chub, when he edited some of the works, are apparently accessible:
Some good news. I found the scans of two Old Slavic manuscripts used by Archbishop Mikhail.
See http://www.stsl.ru/manuscripts/index.php?col=5&gotomanuscript=040, the first two manuscripts (40 and 41) from the list.
Sadly one can’t download the things as PDF’s — they’d be much easier to look at in that form!