Some of my colleagues have suggested waiting for the IRS to notify you about the amount due, in lieu of preparing form 9465.  They indicate the IRS will notify your client if payment of the amount due is not made with the return, kind of like an invoice being sent.  Obviously, penalties and interest will still apply but your client will not be on a set payment arrangement.  Once that "invoice" is received, your client can pay what they are able to pay.  Assuming it is not the full amount, the IRS will once again invoice your client...and so on.  I don't have personal experience with this but several colleagues swear by it.