Has anyon ever seen a tempo like this before?
Unless it means “circa (around) 60 bpm,” I have no idea. Dorico would not accept this.
A symphony hornist showed me this on his part.
IIRC this option has been added in Dorico 6 
I think the only changes in Dorico 6 were to the appearance of the mark, the c./ca. has been around for a long time.
To confirm, yes, c. means circa/approx in this context. Sometimes you’ll also see it as ca. 60
