I measured the ANSI through my ISCO II lens a few years back, and despite the image going through a lens with dust on it, the ANSI was much the same as the pj when measured without the lens.
I think it's a bit of a myth IMHO.
Glass is not 100% transparent, granted better glass is more transparent than ones of lesser quality, dust is a light blocker altogether(dirty sunglasses/reading glasses anyone?)thus light passing through will be affected to varing degrees in reaching it's destination.
Measured results depends on the quality and sensitivity of the measuring instrument.
UMR(Jeff Meier) uses some SOTA measuring gear:http://homecinemaguru.com/?p=698
"I recently aided a test with a Prismasonic H-FE1500R and a JVC DLA-RS35. The projector was mounted at a 2.0 throw ratio. Using lens zoom to fill a 10 ft wide 2.35 screen the ANSI contrast was measured at 250:1. With the anamorphic lens the ANSI contrast fell to 112:1."
and that is just the A-lens.........I would think dust would have a significant effect on light scatter and a decrease in ANSI and light transmission further.