How can I change the loading screen that is displayed between two maps?
I can already create custom loading screen for a single map, by naming it sp_aXX and having a loadingscreen_aXX material in the right directory. But the loading screen that shows up between two of my maps will always stay the aperture laboratories logo, and I find it pretty annoying. Any idea?

You could use Texmod to switch out the game's textures for your own custom ones. If you wanted to permanently change them though, you'd probably have to decompile/recompile the game's VPK files.

I think Reepblue is the only one who has successfully done this for a map with his A Little Higher mappack. PM him about it.

If I remember correctly, there was the same issue with his map pack: always the aperture logo between maps.
I'm afraid there's nothing to do.

Is there not anything in the transition script called by arrival_departure_transitions_ents or whatever it's called that does this? It would seem to make sense... Then you can just create a copy of it under a new name, modify the instance to call that, and change whatever lines of code you need to. Something tells me it wouldn't be that easy, though.

sp_transition_list.nut is used by the arrival_departure_transition_ents instance to know which level is next. But I don't think it chooses which loading screen will be displayed. After several testings, I think that the loading screen between two maps is managed by a code inside the game itself. Am I wrong?

What about the loading images themselfs? If you can locate them you can have your .VPK overwrite those files(by having them in the same file hierarchy inside the .VPK) It should only choose the custom loading screens when accessing the .VPK (when ur playing the custom map)

No, I think the contents of the latest .vpk loaded override everything else; that's why you have to be careful when including custom light board scripts in vpks.