Okay, so I finally put my hands to the work. The solution consists simply in uploading some packages to apt.armbian.com, and allowing users to install them manually.
The packages are essentially the same as the ones provided by the old media script, plus one metapackage to install them all and apply the necessary configs. For example, for the Tinkerboard, we have the following package set:
Libraries: libmali-rk-midgard, librockchip-mpp, librk-vpu
Custom X server: xserver-xorg-core-rockchip
Media players: gstreamer1.0-rockchip, mpv-rockchip
The meta-package: media-legacy-tinkerboard
When the user does apt install media-legacy-tinkerboard, the meta-package installs the whole media solution, pulling all the other packages and doing the necessary system configs.
I am planning to give support to all the boards that I made a media script before, namely Tinkerboard, RK3399, RK3328 and Odroid XU4.