use non-default pixel format?

[bbcuffer]

This could be me rather than av. Is there a way to specify the pixel format to be used by the video codec?

Trying to output ProRes with an alpha channel using format yuva444p10le

av sees the prores_ks codec:

> av::av_encoders() |> filter(name=="prores_ks")
   type      name               description      format
1 video prores_ks Apple ProRes (iCodec Pro) yuv422p10le

and ffmpeg sees the yuva444p10le format for that codec:

$ ffmpeg -h encoder=prores_ks

ffmpeg version 7.0.1 Copyright (c) 2000-2024 the FFmpeg developers
  built with Apple clang version 15.0.0 (clang-1500.1.0.2.5)
  configuration: --prefix=/usr/local/Cellar/ffmpeg/7.0.1 --enable-shared --enable-pthreads --enable-version3 --cc=clang --host-cflags= --host-ldflags='-Wl,-ld_classic' --enable-ffplay --enable-gnutls --enable-gpl --enable-libaom --enable-libaribb24 --enable-libbluray --enable-libdav1d --enable-libharfbuzz --enable-libjxl --enable-libmp3lame --enable-libopus --enable-librav1e --enable-librist --enable-librubberband --enable-libsnappy --enable-libsrt --enable-libssh --enable-libsvtav1 --enable-libtesseract --enable-libtheora --enable-libvidstab --enable-libvmaf --enable-libvorbis --enable-libvpx --enable-libwebp --enable-libx264 --enable-libx265 --enable-libxml2 --enable-libxvid --enable-lzma --enable-libfontconfig --enable-libfreetype --enable-frei0r --enable-libass --enable-libopencore-amrnb --enable-libopencore-amrwb --enable-libopenjpeg --enable-libspeex --enable-libsoxr --enable-libzmq --enable-libzimg --disable-libjack --disable-indev=jack --enable-videotoolbox --enable-audiotoolbox
  libavutil      59.  8.100 / 59.  8.100
  libavcodec     61.  3.100 / 61.  3.100
  libavformat    61.  1.100 / 61.  1.100
  libavdevice    61.  1.100 / 61.  1.100
  libavfilter    10.  1.100 / 10.  1.100
  libswscale      8.  1.100 /  8.  1.100
  libswresample   5.  1.100 /  5.  1.100
  libpostproc    58.  1.100 / 58.  1.100
Encoder prores_ks [Apple ProRes (iCodec Pro)]:
    General capabilities: threads 
    Threading capabilities: frame and slice
    Supported pixel formats: yuv422p10le yuv444p10le yuva444p10le <<---- HERE
.....

But can't see a way to tell av to use that format. e.g.

> av::av_encode_video(anim_fns, 
    output = "demo.mov",
    framerate = 25, code = "prores_ks", vfilter = "format=yuva444p10le"
  )

gets me video encoded in the default format

> av::av_media_info("demo.mov"))
$duration
[1] 6

$video
  width height  codec frames framerate      format
1  1920   1080 prores    151        25 yuv422p10le

as does vfilter = "format=pix_fmts=yuva444p10le"

Is it that I haven't specified the pixel format correctly or established whether it is, in fact, available for av?