Maybe it would be most expedient to just ignore bounding boxes and force all catenary_back sprites to be drawn under any vehicles?

As a reminder to who-ever picks this up: some GRFs, like CZTR TRAM SET draw the catenary from both sides.

I gave it a spin to fix, as can been seen by the above PR, but .. I found out that there are some .. special choices made for trams :) Basically, some sprites in OpenGFX have pylons on both sides of the track. In general, for isometric games, if you have a single sprite that has to go both over and under another sprite, there is no correct way of sorting that. Unless OpenTTD has tricks up its sleeve to deal with these situations (my knowledge about this is severely lacking, so I hope someone else can fill in that gap), there is no answer to this problem.

So I am afraid you will have to deal with this problem a bit longer ... sorry :(

Edit: okay, gathered more information. Turns out the idea is that the back sprite draws the pylons, but the front sprite does this again for the pylons in the front. This is currently not done in OpenGFX, which was confusing me. It is fine that the back sprite has the pylon too, of course, but the front sprite should at least have it. Otherwise it is indeed an impossible situation for sorting .. but if the idea is that both have them, nothing is wrong here. It is just that OpenGFX got it wrong.
The tramtracks.png in our source tree does seem to get it right, so there is that.