Any chance you could add support for assumerole auth? That is, have a single account (probably a Orgroot), but then for each addtional account, use a named assumerole to pull data from the other accounts. 

That was, you could have a set of canned roles for running Inventory across a wide number of accounts, but only need the credentials from the Primary account. And let IAM assumerole configuration allow actions for the other account.