Looking at the func_pendulum c++ class, shows that it doesn't care or use angles var for anything. So the only thing I can think right now, to solve this, is to bind the visible entity, to a invisible pendulum mover, moving in the direction you want the visible one to follow, its own axis doesn't matter (visually but do matter to the pendulum movement), you can even make it a brush box. Hope you get what I mean.
Also here is a old Doom 3 pendulum tutorial that I found, I didn't bother to search in TDM wiki, so perhaps there's one there as well, but just in case see if this helps.
https://web.archive.org/web/20100522163241/http://www.doom3world.org/phpbb2/viewtopic.php?p=84076#84076