Yes you can do it: Sigma BF with the massive 300-600mm lens :)
At the CP+ show a Japanese enthusiast tried out the new BF camera in combination with the massive 300-600mm lens 🙂 I would say that we need a proper bigger Sigma camera for that to work out fine!
300-600mm at BHphoto. Amazon. Adorama. FotoErhardt. Fotokoch. WexUK.
16-300mm at BHphoto. Amazon. Adorama. FotoErhardt. Fotokoch. WexUK.
Sigma BF L-mount camera at BHphoto. Amazon. Adorama. FotoErhardt. Fotokoch. WexUK.
Source: masakiishitani