Katherine has already given you the solution to sync the entire database, but to elaborate further…
Yes, the alpha test group did work on this. It might be worthwhile to keep in mind that the most advanced iOS hardware when DEVONthink To Go was in early alpha was the iPhone 3G. The iPad had not even been announced yet. The idea behind DEVONthink To Go was that users could take a sub-set of their databases mobile on the iPhone. Even with the iOS hardware available today, this is still the only viable option for those of us with larger databases.
You cannot replicate documents between databases-replicants can only be created within a database. The only way you could create a mobile database would be to duplicate the documents to that database. I would not consider that a viable solution at all as a) the duplicates just take up twice as much space and b) changes to the documents in the mobile database would not be reflected in the other databases.
To get exactly what you want, just select all the top-level folder groups in your database, right-click on the selection, and choose Replicate to, and select the Mobile Sync group. At this point, your entire database (minus the database inbox) will sync to DEVONthink To Go.