Have you considered registering as an enterprise developer? It's a bit hard to work out what the limits are on it, but it may be possible. Getting a DUNS is free, then it would only depend on how strict Apple are on proving you have 500 people in your organisation. I would assume that enterprise development is open to non-profit organisations as well, so you could register Litchie as the "non-profit, iphone/dos appreciation society" and try and get 500 members, who just happen to donate an annual fee to the Litchie organisation, which just happens to provide an app to its members, which just happens to run DOS on an iPhone
I'd certainly recommend something like testflight
if you are doing beta testing. It provides it's own app that sends the phone's UUID to you automatically, and provides easy notifications to the testers for new versions. I don't know how many actual users you have, but would it also be possible to create a number of "different" iDos apps that are in fact the same app with different names, and then push each one out to a different group of 100 testers?