About newbies having to find "secret" functions, as I said in the other thread, Simutrans just need built-in tutorials in each pakset. There's no easy solution, Simutrans is a complex game and it should stay like that, if we want newbies to understand everything in the first hour of playing we might end up with a facebook-like time-wasting moron game. So "secret" functions are not that bad, especially if they are not essential in the game.
The current GUI of Simutrans is not perfect, especially menus with so much icons. I remember when I first played Simutrans, even pak128 had very few icons in its menus. Back then the GUI was okay. But now menus have too much icons, even experienced players sometimes strugle through menus to find what they want. Until someone comes up with a more efficient GUI (it's not that I don't have ideas about this, but I still have to dive in the code :p ), we should avoid adding too much icons. Cycling through more than three signal positions would be a hasle too, and even more annoying one than adding icons to toolbars.
Then, if you really want to have the choice of which type of signal you want, the best solution for now is still adding icons.
Anyway, in case someone wants to code, here is a simple idea. When we want to build station extensions that have multiple orientations, we ctrl+click the icon to choose. Why not doing the same with signals ? Let's have one icon for all types of signals, which would change automatically according to the place they are built in. Then, if you want to choose what graphics you want, ctrl+click the icon.