I'm still not quite sure if this is properly reproduceable, but i managed a basic minecart switch that works with a perpendicular roller, without constraining walls or anything, just by the magic of carefully regulating speed and building lots of track corners to catch and re-regulate the diagonally moving cart.
Behold:
Buildings:
The carts enters at medium speed (coming from the roller to the left), the actual switch consists first of a medium speed E->W roller on the NE track corner. Depending on whether this roller is on or off, the cart is sent around the loop or passes straight east. Thus it encounters the roller in the centre, a highest speed roller which works S->N, either coming from the west or from the east. If the perpendicular roller is off, the cart simply passes over it and leaves on one of the south-leading tracks - either the right one when both rollers were off, or the left one when the first roller was on and the perpendicular off.
If the perpendicular roller is on, the cart is sent on a 'high-angle' diagonal trajectory, either NE (first roller off, perpendicular on) or NW (both rollers on). It is then caught by the SW or SE track corner two steps away from the perpendicular roller, and because of the trajectory angle (i think; it works less well or not at all when all involved rollers work at the same speed) gets 'caught' by the corner and returns to obeying track paths. The NE-going cart moves off on the rightmost north-pointing tracks, the NW-going one ends up on the "half-left" track. The one all the way left worked when both the first switch roller and the perpendicular roller were highest speed. Most of the tracks to the north is complete nonsense, as far as i can tell only the 'catching' corner and an outgoing track are needed for each switch option.
The main selling point of this device is of course that it takes only two rollers to convert the four possible combinations of two 'bit' inputs into four distinct 'outputs', something that takes a full _three_ rollers otherwise.
BTW, the roller to the northwest (north of the "starting" roller) sits on a non-floor tile and wouldn't accelerate a cart even if one passed over it, i used it only for power transmission. This way, the entire circuit requires at most exactly the forty power produced by the two windmills. The roller cost a rope extra, but saved a crucial three power over a gear assembly.