I figured out a workaround: Use iridium-browser instead. I prefer iridium anyway, as it's more secure by default.
WARNING: What follows is a horrible, horrible hack. I'm just too lazy to do things properly when I'm the only one who'll be using it.
1. Download the latest iridium browser:
$ cd $HOME/Downloads
$ wget https://downloads.iridiumbrowser.de/deb/pool/main/i/iridium-browser/iridium-browser_2019.04.73-1iridium0_amd64.deb
2. Extract the contents:
$ tce-load -wi flex bash # flex is needed by ar, bash is needed by iridium-browser itself (shame on them)
$ ar x iridium-browser_2019.04.73-1iridium0_amd64.deb
$ xz -d data.tar.xz
$ tar -xvf data.tar
3. Create a tcz extension from the contents:
$ mkdir root
$ mv ./etc ./root
$ mv ./usr ./root
# strip everything to make smaller:
$ find . | xargs file | grep "executable" | grep ELF | grep "not stripped" | cut -f 1 -d : | xargs strip --strip-unneeded 2> /dev/null || find . | xargs file | grep "shared object" | grep ELF | grep "not stripped" | cut -f 1 -d : | xargs strip -g 2> /dev/null
$ mksquashfs root/ iridium-browser.tcz
4. Put things in your tcedir/optional/ directory:
a. Copy iridium-browser.tcz to tcedir/optional/
b. Grab chromium-browser.tcz.dep from the TC repository, rename it iridium-browser.tcz.dep, add bash.tcz to it, copy it to tcedir/optional/
5. "Convert" your chromium profile:
$ cp $HOME/.config/chromium $HOME/.config/iridium
6. Enjoy!
$ tce-load -i iridium-browser
$ iridium-browser
I'm happy with this workaround until a proper solution comes around. h264 support must be baked-in, since h264 videos play just fine even without ffmpeg4.tcz loaded.