Same problem here for more than a month now. Contacted both Adobe and Blackmagic Design, but neither came with a proper solution yet.
HD compositions do output, but SD does not output on HD monitor via Blackmagic nor via HDMI
It still works in AE CC, but not in AE CC 2014.
I think Mercury Transmit lacks the scaling features? Or it just does not address this the right way to the hardware?
Please come with a fix for this, since broadcasters are still requesting SD programmes.
http://www.jaydude.nl