Is it possible to use 2-pass encoding to create vp9 / webm videos from an image sequence?
You would use

ffmpeg -framerate 30 -f image2 -i frames/%04d.png -pass 1 -c:v libvpx-vp9 -pix_fmt yuva420p -b:v 0 -tile-columns 2 -crf 18 -r 30 -g 300 -y -f null -


and then

ffmpeg -framerate 30 -f image2 -i frames/%04d.png -i my_audio.flac -pass 2 -c:v libvpx-vp9 -pix_fmt yuva420p -b:v 0 -tile-columns 2 -crf 18 -c:a libopus -b:a 128k -r 30 -g 300 -y my_video.webm


In FFmpeg the container format or encoding of the input is, in itself, irrelevant in terms of using encoder features like two-pass.