Would a restriction endonuclease cut messenger RNA?
Restriction endonucleases are highly specific in relation to their "target" DNA. Most of them have a go at double-stranded DNA (dDNA) where they recognise and act upon palindromic sequences.
In ssDNA (single stranded) or RNA that is a bit difficult, as there is no palindromic site. However, as it is ss, the strand might (and usually will) coil back on itself and match two regions in the strand that might form a palindrome. tRNA's (Transfer-RNA's) are a good example.
DNA-targeting Endonucleases probably won't act on RNA anyway due to the presence of Uracil instead of Thymine..
Nevertheless, some RNA Endonucleases HAVE been identified. For more info, have a look at: https://www.ncbi.nlm.nih.gov/pubmed/18217203