Consolidated Billing at Party Level

Hi,

I have joined a client where customers have been setup as one account to one site i.e. no multiple sites into one account.  They now want to use consolidated billing (or balance forward billing) across the accounts which I know is not possible.

Has anyone produced an extension that can cater for this or provide any thoughts into the process that should be adopted?

I am thinking of holding the invoice number at Party level although this then causes an issue for cash reconciliation and will require an additional extension.

Any thoughts welcome

Thanks

Jon