As far as I know (and I don't know everything!) the only all-Myrtle steel-string guitars existent are from Breedlove. There are probably others.
I have had good luck with my previous (nine?) all-hardwood guitars, as their owners will attest, and spectacular results with my Myrtle back'n'side guitars. Thus it seem like barely any risk at all to combine the efforts into an all-Myrtle guitar and have a reasonable expectation of something truly special.
Yesterday I put the first coat of varnish on CN#4, the guitar of the moment. Like my previous Myrtle work, I have treated this with my rendition of the Cremonese Violin maker's varnish secret, which seems well suited to Myrtle.
