I don't understand why you're caught up about rendering "uncompressed". Your 8bpc H.264 copy will by definition be much lower fidelity, so even at the best quality settings any pixel differences between the MP4 and the raw timeline frame will overwhelmingly be due to the H.264 compression itself. Yes if you repeatedly re-encode a clip with H.264 it will degrade, but unless we're talking about some scientific research project then a Cineform or DNxHD export can be treated as visually lossless. Put the export on top of your composition and toggle the layer, I challenge anyone to see a difference (provided you didn't screw up the color management).
Exporting from AE once to a mastering format, then transcoding the file into deliverables, is standard industry practice. Rendering all the versions from AE is a monumental waste of time; After Effects was never designed to be the end of the pipeline and frankly if you want the very best H.264 compression quality don't use Adobe software at all.
Many studios work with ProRes intermediates for historical reasons but the results with the right flavors of CF/DNx will look exactly the same. If you need pixel-perfect replication then you always render an image sequence (EXR is the usual choice), but that's not something you can upload to InstaBook...