I have run no experiments, but based on my reading this should not work effectively for mixed yeast/bacteria cultures. Apparently the two require different environmental treatments to induce them to hibernate so processes to get both in dried powdered form are not as simple as just air drying out at room temp.
Matthew, I appreciate you posting this topic & I think that it would probably work most of the time. But scpizza does have a valid point. I'm not claiming to be an expert (with only two courses in microbiology), but the literature does indicate that while yeast survives the drying process fine, something like 99% of the lactobacilli will be killed with simple drying. And you need both the yeast & lactobacilli to have a stable starter culture. Your lacotbacilli may recover over a few weeks, but as almost all of the lactobacilli are killed, it's not guaranteed that another strain of bacteria might not predominate when you reconstitute your starter. Certainly this was a problem in the past commercially and due to sometimes uncertain results, simple drying/freeze-drying is no longer used commercially.
I have no idea what sourdough.com or other vendors use for a process, but assuming they follow current commercial practice, they would do something along the lines of concentrating the lactobacilli in sterilized milk, ph adjust (neutrialize), add stabilizers such as calcium citrate (easy to find in a drugstore), tween 80 (a little more esoteric, so mail order), then freeze dry to preserve a much higher concentration of the lactobacilli. This is not out of reach of the hobbyist, but it is a bit more than most would attempt.
It's difficult to say if this is a real concern; I would guess
that the majority of starters would recover. But it would not surprise me if it turned out that a significant fraction of reconstituted cultures had problems. A simple search didn't pull up any quantitative data for this. For short term storage i.e. a few months, it does sound like simple freezing in a decent chest freezer works fairly well. For anyone interested, there is a better discussion of this in Yoghurt:science and technology
, starting at page 487.