I don't think it was very difficult when I tried it. Just grab the textures and main mod, put them somewhere (e.g. .local/share), and then append the following to your openmw.cfg config AFTER the base game:

data="/home/user/.local/share/mw/Tamriel Data (HD)"

data="/home/user/.local/share/mw/Tamriel Rebuilt"

Ah yeah, I remember that being pretty hard. It has been a long time since I set up TR.

I think the tricky file to find was ~/.config/openmw/openmw.cfg and it needed to che changed to include:

    fallback-archive=PT_Data.bsa
    fallback-archive=TR_Data.bsa

and

    content=Tamriel_Data.esm

Then everything else was managed with the openmw-launcher UI. The game ended up installed in ~/.local/share/openmw/ with some funky setup to deal with mods.