Hi Geoff,
I believe you are confusing some issues here. Address lists in Exchange (and as a result the generation of OABs) are not based on smtp domains. Each Address List is a grouping of Active Directory (mostly user-) Objects based upon certain criteria and - in the case of HMC - "seperated" via permissions. OABs are generated and updated (scheduled proccess) based on these Address Lists. Each user object in turn can have (individually) one or (many) more smtp addresses associated with it - regardless to which Address List the object "belongs".
That being said, HMC is a little more straight forward in that it is assumed that virtual Organisation A and virtual Organisation B will most likely not share an smtp domain.
As far as Exchange scalability goes, I don't recall any mention of an smtp domain limit - I did read something about a recommended maximum for OAB hosting/generation (in addition to when to set up dedicated OAB boxes). I believe that was somewhere in the HMC documentation, but Technet also has a lot of information on setting up large organisations (Exchange organisations in this sense).
Sounds to me like you would be best served by contacting MS directly to get some scalability advice.
HTH.
Rgds - Marcus.