Unfortunately, this week I do not have anything ready for you. It is with great regret that I must therefore skip this week, and break my “once-a-week” posting schedule. Regular scheduling should return next week.
Just a quick note, however. Thanks to Sylvain Prévost bringing this back to my attention; Brûlet et al. derived a very similar equation to what was derived in last week’s post. Additionally, Strunz et al.  have some additional considerations for transmission factors that may need to be considered in X-ray scattering as well (in particular for ultra-small angle X-ray scattering).
Hopefully I will have the time to look into these things in the near future and give you some more insight on the magnitude of these problems. As suggested by Sylvain, it may be a good idea to adapt the “imp2″ data reduction software to be able to handle a more general consideration of transmission factors (then supporting both SAXS and SANS). A bit of thought is needed on how to enable fancy background subtraction while keeping the modular, flexible nature of the program.
: P. Strunz, J. Scoversheet.dviaroun, U. Keiderling, A. Wiedenmann and R. Przenioslo, “General formula for determination of cross-section from measured SANS intensities”, J. Appl. Cryst. 33 (2000) 829–833. [journal link]