The screen on the Seedhouse SL series is pretty light to begin with, so it probably isn't quite the 32% that Bill mentions. I've got a sample of screening that feels very similar to the mesh used by BA, and it weighed out to .97 oz/sqyd. Nanoseeum is quoted at .71 oz/sq yd so you save about .26 oz/sq yd.
How did you extrapolate the weight of the screen as 18oz.? Using the proposed weight above, that would mean that the BASSL would have roughly 18 sq yds. of screening. That seems a little high to me and is probably more like 4-5 sq/yd.
With that, you would only save 1-1.25 oz total. The material cost wouldn't be that high (~$25 for screen, plus some extra for new grosgrain for the pole clips), but it seems like a lot of time and effort to save such a small amount of weight.
On top of that, the screen on the BASSL has been noted as "picking" easier than standard screen mesh. If nanoseeum is even more fragile, than it may be one more reason not to try the project.