Well, presumably you'd be using the Steam Beta system to give access to the beta? There's a way that you can request beta participation, so I figure that people who have Dragon Commander in their Steam library would be allowed to request participation in the beta, and then they'd have the beta app in their library too.
I don't know if you could do a default opt-in sort of thing, and you'd have to deal with the people who shouldn't have access to the beta (since presumably post-release the beta will be for purchasers only.)
I think however it works, some segment of people will have to download more than they want to download.
(edit: but a preload would be nice for those with slow internet to get a head start.

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