Well done for having a go. I'll note some more tech details...
What I ended up doing was to add a whole bunch of light sources in a line going vertical. Like a vertical fluorescent strip light in the direction of thatching and I created a single image render made from the brightest parts of those light renders. I used directional light sources not point or spot. Then I clamped the lighting so that only the very brightest parts of the image stayed white and all else went black. That basically made only the parts of the model which faced in the exact horizontal (X-Z plane) direction of the thatch white but at any vertical angle (Y angle). So for example a ball like an orange would have a segment lit white and all else black.
So I had 8 image masks and an extra non directional thatch texture. The masking and compositing is done with a compositor that I have as part of GTS but the order I used was as follows.
Start with an image which is all floor color - say black.
Mask in the non directional thatch where the object is not floor only. That is to say if the height map is not black then render the non directional thatch.
Then multiply LightMask7 (157.5deg) with Thatch7 and add in.
Then multiply LightMask6 (135.0deg) with Thatch6 and add in.
...
Then multiply LightMask0 (0.0deg) with Thatch0 and add in.
And that should be it. So Thatch at 0.0 deg is the highest priority and the non directional thatch is least. I tried doing odd angled first like 22.5, 67.5 and then the 45's and then the 90's and then 0 but that didn't come out as well as doing them in rotation order. You could end up with small patches of one angle where everything around it was another completely different. By doing them in rotation order it seems to blend the angles better but the down side is that if you get the light clamping wrong then the bits of thatch which would be 90deg and well lit by the 90deg light mask could be wiped out by the overlap from the 22.5 Then when you get the final render all the angles are a bit out.
I accept that this is not the easiest thing to set up but once you have it it seems to work time and time again.
To get the non directed thatch I merely took 0 & 90 deg and put it in two layers in the bitmap editor and blended 50:50. I was hoping that I wouldn't have too much of that showing really. You cant have thatch pointing straight up as it would let the water in ! I don't know what real thatching does on the top of houses. I guess they put some flat thatching across. Sometimes you see these great bits like topiary with a thatched peacock or something very clever.