I'm not so sure Mark. It would need more market research. Most of the toys that rate well here and other places tend to be investment pieces costing $75+. Given that I would think it would advantageous for a company to have the ability to give the customer as much information to determine their purchase as possible. Plus, depending on the accuracy, a store could charge for this service and would likely have the ability to upsell the customer as it would more then likely marketed as free if you purchase a certain level of product. It's not like it's a novel idea. I have been to Adult stores where you will be charged for browsing if you do not purchase an item and I have been to bra stores that charge for fittings.
Perhaps there are too many things to consider in using just a "sample model" if you will, so maybe going with a 3D measuring system would be more feasible. It would also address toxie's concern that the vagina changes during arousal. While the scan was being done the customer, they could be shown arousing images or be stimulated non vaginally either by a partner or themselves which would allow the changes to be recorded. Granted, this seems that it would be invasive for some, but then again I have seen people go to great lengths for customization.
I am likely over thinking this, and there is probably a simpler solution out there. It's just as someone that has been burned more then once on expensive toys that didn't fit and as a person that likes to have as much information as possible when making a purchase, I know that there has to be a way to make toy shopping a more reliable experience.