FFmpeg - feeding output of encode operation to filter

I wanted to know if we can feed the output of an encode operation to a "filter_complex" with a command like:

ffmpeg -i input.mp4 -c:v libx264 -b:v 10000k "[encoder-output-1]" \
-c:v libx264 -b:v 5000k "[encoder-output-2]" \
-c:v libx264 -b:v 2000k "[encoder-output-3]" \
-filter_complex "[encoder-output-1][0:v]psnr" -f null - \
-filter_complex "[encoder-output-2][0:v]psnr" -f null -\
-filter_complex "[encoder-output-3][0:v]psnr" -f null - 

If we can do something like this, how should one name the output pad of the encoder, so that one can reference/map it in the filter_complex If not, please let me know what is the easiest way to achieve something like this.

Note:

1. I would be using third party encoders that don't have the capability to calculate PSNR scores internally. Thus, I would like to compute the PSNR within an FFmpeg filter.