Did you ever seen MM flows without implementing MIRO ?
Currently, the customer hasn't got MM, only FI. And he is posting a complex accounting document using FI invoice t-code, which feets to his need.
If we try to adapt MIRO to do the same posting by user exits, it will be very complex (and I don't think, it's possible).
What do you think about implementing MM flows, but without doing the invoice verification in MM : ie maintaining the invoice control in FI transaction (the creation of this FI document will be done automatically by interface from another system) ?