First, Old Church Slavonic:
Old Church Slavonic differs from Rohoncian in that the most common letter is more frequent as a final than as an initial, and there is a wide disparity between the frequency of the second letter as initial and final.
Second, Koine Greek:
Koine Greek differs from Rohoncian in that the third most common letter is more common as an initial than as a final (though if the iota ended up in third place, it would fit well, with initial and final frequencies of 2.7% and 8.6%, respectively).
Suppose we give each of these languages a location in six-dimensional space, indicated by the relative frequencies of the top three symbols as initials and finals...what would their distances from each other be in this space? And which would be closest to Rohoncian?
Interestingly, the two languages that are closest to each other by this measurement are Rohoncian and Latin, with a distance of 0.115. Next closest to Rohoncian is Old Hungarian, with a distance of 0.130. The languages that are most distant from each other are Koine Greek and Old Albanian, with a distance of 0.293.
Overall, I am weakly inclined to think that Rohoncian is some kind of Latin or Hungarian. Not only does this particular measurement favor these two languages, but there are graphical similarities between the Rohonc C, I and the Latin e, i.
(In case you are wondering, I also looked at Voynichese, just for the fun of it. It differs significantly from all of the other languages I have looked at, in that the second and third most common letters occur infrequently as initials or finals.)