It appears that Rokinon (also known as Bower, Samyang, walimex pro) is about to release a 300mm f6.3 mirror lens for APS-C-sensored mirrorless interchangeable lens cameras, such as the Sony NEX and Fuji X series. While we have no official confirmation from the company yet, the pictures appeared on its facebook page first, so the news seems legitimate. There are little details known so far, but Photography Bay states a retail price of US-$ 299 and a release date sometime in June.
Tokina is already offering a similar lens, albeit for Micro Four Thirds, with exactly the same specs and price tag. We reviewed that lens a while ago. While I don’t know the exact imaging circle of the Tokina lens, it is entirely possible–considering it is all manual in operation–that it can be adapted to APS-C without loss in image quality.