Very interesting! I've seen that explanation too: if I recall, in an Appendix to Chambers English Dictionary, a pretty old edition (1960s perhaps). Not sure if they appropriated it from the Oxford Dictionary of English Names, but they might have. My assumption was that the hanging of roses on tombs was more likely to be pagan than
Christian, but my Latin dictionary (Cassell's) doesn't give it.
Rosalis, or Rosalys, turns up in a 19th century poem called The Blessed Damozel, by
Dante Gabriel Rossetti: there, she is one of the handmaids of the Virgin
Mary 'whose names are five sweet symphonies', but only gets a passing mention.
I look forward to more input!