Ok, I'll go and dig out the little bits of scrawled on paper I made during my search.
Hunter-Douglas appears to make some intersting materials. These are used in their "vignette window shading series". Specifically look for the "Satin-weave" materials. There is a excellent white fabric in use there which is quite strong, and quite opaque seeming and with excellent sound transmission. A difficult combination to come by. High opacity allows simpler solutions when having to implement a shadow box. This appears to very, very promising. Orientation of the weave for viewing purposes is the major potential problem here. It has minimal intrusion in this respect and may not turn out to be a problem at all.
Check this out.
Shade-o-matic corp. (may be a canadian company) They have a plain "roller shade" design with "sheerweave 4000 or 4400" (not shure about the number) used as a base material. It appears to be the same as, but slightly different design compared to a well known acoustically transparent screen material out there. A shadow box is definitely needed with this stuff, if the hole size is correct enough (not to big, not to small) for this kind of usage. There is an interestingly large number of possible solutions in this range of product. If you go ahead and decide to experiment, bring some sort of white reference with you so as to make sure you get the right white, there are a couple of variations.
While I was there, I requested a price quote for the blind. (the shad-o-matic) I said a size of 90" wide by 50" high, just for the heck of it. They were 45% off at this time. The final number came out to $335 canadian for it. That's about $225 or so to you yanks.