Blackmagic does not currently support scaling frames from After Effects or Premiere Pro that do not directly match one of the card's supported frame formats. For example, Blackmagic cards support standard frame sizes such as 720p59.94 or 1080p24, or up to 2K or 4K depending on the card, but it wouldn't support an arbitrary comp size and frame rate like 999x999 @ 99fps.
Automatic scaling is a feature of Mercury Transmit (which is different from the Mercury Playback Engine, which is not in After Effects), which other video devices do support. When After Effects sends a frame that the video device doesn't support, the video device has the option of asking After Effects to scale the frame to a supported format. Blackmagic does not do this.
We are discussing this issue with Blackmagic. They will need to update their plug-in for Mercury Transmit to support this feature.
I hope very much that your discussing with BM is successful. I know they can do it and they should because AE is such an important application!
I was in touch with BM and they confirmed that Scaling on output would be something for the future with no date mentioned...
Just to mention: SDI Output scaling in AE 2014 works perfectly fine with an old AJA KONA 3 !
Finally Blackmagic updated their Desktop Video Software to make it "customizable" in AE 2014.
Well the setup dialogue in AE video preferences gives three options:
No output: which means "off"
center: which cuts off the picture if you are working with material wider than HD (like Scope, or everything that needs to be letterboxed)
scale to fit: which distorts the picture awfully because it fits everything into a wrong aspect ratio
What we would need is just what is in AE:
Like "fit to width" which would maintain the right aspect ratio and scale a scope into HD.
This should be possible I think. Otherwise nesting into a HD sequence is the only way again.
(DaVinci does it, easydcp does it, all with the same BM card)
I've just now had an opportunity to test the most recent Blackmagic drivers, version 10.1.4. I see the scaling options that they've implemented. I don't think this takes advantage of the automatic scaling options that are available via the Mercury Transmit API, and which are implemented by the other manufacturers and would do as you suggest. I will ask Blackmagic for more detail about how their new drivers do scaling.