All,
I have followed this discussion from its start and am constantly amazed at what the internet allows a few motivated people to do. I just love these kinds of globehopping collaborative projects.
In reading the latter part of the thread and the problems with flare and glare, a question keeps popping into my head; Why do you need to measure a paper target? Why not take a variable light source like the LEDMOTIVE or a homemade alternative (RGB+ LEDs and a ColorMunki?) point it at the camera and take photos as you vary the light? I guess it would look more like a version of display profiling. On that thought, maybe instead of LEDs it could even use a display for the source?
It's probably harder and more expensive than we'd like to make a home-made programmable multi-channel LED light-source. I'm all for such a project though, but I'm not the right person to do it.
Targets should present relevant types of spectra. Homemade inkjet printed targets have some limitations discussed earlier, X-rite's targets have better spectra, but I think you can use a inkjet super-saturated target as a complement to X-Rite's matte CC24 for example. If you do copy work, such as copy paintings, making an own target by painting patches with the same colors as used in the work you copy will be a good idea.
The core of the problem is that cameras are not colorimetric devices, their SSF do not match the standard observer CMF, that is they register colors in a significantly different way than humans. This means that to make a profile that works as good as it can, we need to make it for the exact reflectance spectra that will be in the scenes we shoot. Unless we do copy work this is not really possible, and then we instead need to do it for some average spectra, then smooth rounded spectra are much better than spiky spectra as it's more similar to natural colors and works smoother over a wider range of similar colors.
Unfortunately displays are very limited in terms of spectra, it's just a red, green, and blue peak which you can mix. This can recreate tristimulus values that work well for a human, but as the camera sees colors otherwise those spectra will not make up for good profiling colors.
It's the same reason why we're so picky about the artificial light sources. The mismatch between human and camera color response makes spiky spectra unpredictable. If you have photographed in low CRI light sometime you have probably noticed that the colors come out quite different from the camera than the eye saw them, and this is a demonstration of the effect of having different color response. The spikier spectra the more the differences are seen. Therefore we want to do camera profiling with full spectrum lightsources with smooth spectra, using colors with smooth wide reflectance spectra.
Using transmissive targets is interesting though. In Image Engineering's CamSpecs product they use transmissive targets with several narrow bands to measure the camera's SSF. And I think if one is going to make some elaborate DIY project with hardware stuff, I think one should try to look into measuring SSF.