@Christian:
What you are looking for is ZBrush. I have been looking into it myself a while ago and was quite impressed. It’s not too expensive and one can get results within a day or two. There is a tool that does exactly what you are looking for - you can color the high detail mesh in areas where you want less simplification. I can’t imagine RC implementing something like that - you can’t have everything in one program and RC is doing what it is supposed to well enough. There are also many core features that need improving… 
I personally decided that the in-program simplification is actually doing not bad at all - it doesn’t just smother everything. Edges are preserved and plain faces reduced more, within a certain range of course. I t also depends on the object. In your case I wouldn’t worry too much. You will have to try a bit for yourself, simplification is not a precise science… 
If you have access to Agisoft, I find their simplification tool is not bad either and differs a bit in the resultiong details.
Just out of curiosity: What accuracy exactly are you looking for? Are we talking about centmeters or half meters? Since it is - I presume - a dirt track and will be shifted slightly with each race, I can’t imagine you want to be on the cm side. How big is your area and how much is covered by one image, meaning what measurement does one pixel have aproximately? Did you use drones?