@Mootjuh I like that you are quite specific with the question. I think that we have already implemented everything that you have described. Pleas take a closer look at the second video LINK , there I place a sidewalk but the same can be done for any kind of road marking (lane, crosswalk, arrows…).
As you can see I can slide it along the road segment and also specify its longitudinal extent (and even just tick one box to make it extend the full road length). And yes, we can make them occur every 300cm or so, but that is demonstrated for the street lamps in the video (see the second half of it) - you can imagine that the same can be done for markings.
I will try to make additional demo video for the markings, later today or tomorrow…