I seem to have found an easy solution to the windscreen shader problem (at least for the SimDream 1937 cars), thanks to the ever-resourceful
@racinjoe013, who reminded me that shaders can be swapped pretty easily via ext_config.ini files. So I created an extension folder with a simple ext_config.ini in it for each problematic car. For the Auto Union Type C (f1_1937_autounion), for instance, the contents of the config file are:
[SHADER_REPLACEMENT_...]
MATERIALS = 37AU_GLASSGRAY-MAT.TGA [replace this with the proper material name for the specific car]
SHADER = ksWindscreen
BLEND_MODE = ALPHA_BLEND
This is working perfectly for the f1_1937 cars, though it seems to be a more complicated proposition for most of the HSS cars with pre-existing configs...I'll keep plugging away on those. Anyway, many thanks to you guys for the help.