I recommend rendering and exporting from After Effects to a lossless master video in a different format (e.g., QuickTime with Animation codec), and then using Adobe Media Encoder to convert that to a PNG image sequence in the background. Adobe Media Encoder tends to be much faster at the encoding phase of processing. You can even use a watch folder in Adobe Media Encoder to make this a one-step process for you---i.e., you just have to initiate the operation in the After Effects render queue, and Adobe Media Encoder will pick up the resulting file from the watch folder and do the next step automatically.
I appreciate your attempt for a workaround solution, however as you well know you can't render to a video file from multiple machines. we must output directly to individual frames, I setup 8 machines to work on this project, if we did not use compression then we would be writing over 300MB/sec of data which would not be good for our fibre network which unfortunatly is only 4Gbit. And it would be about 10TB of data total.
As for your workaround Im not sure we could gain speed with your method either way. spending the extra time to encode to a lossless format and then process again in AME sounds like it would take even more time, unless AME could create PNG files at some sort of ridiculous speed maxxing out every core at 100% or something to compress the PNG's. Out of curiosity I will throw some frames at AME and see how long it takes to compress and how much cpu it will use.
Just did your test and dropped a 4096x4096 pic into a sequence in premiere and exported to PNG and queued it in AME. it was taking 9 seconds to save each frame. CPU usage was negligible, about 4-6% total.
Why don't you use TIF's with LZW compression, it alos has quite small file size!