Nope. AE will always decode all footage to a full uncompressed RGB buffer, regardless which CoDecs you use. You cannot directly manipulate things at the stream level. Re-compressing is inevitable with all the downsides liek having to use other programs or loosing quality.
Why must the rendered clip have the same file size? That way you will work on a compressed clip that you then recompress again and deliver so that the final output can be compressed again. That's three lossy compressions; guess how the final output will look!
Instead: use lossless compression in the production and only output to a lossy compression format. DV is highly compressed and each recompression will destroy the image quality!
I'd recommend that they deliver a QuickTime PhotoJPEG @100% or a QuickTime Animation @100% (which supports an alpha channel). Alternatively you can use the BlackMagic Decklink QT codecs for 10-bit lossless compression. You can then render back to the same codec and you will not cause any qualityloss during the process.
- Jonas Hummelstrand