Look at the second part of this blog - Transaction-free lookup with Personas. There I use a "list display" transaction to do some fuzzy matching and display a list of possibilities, allowing the user to choose one. If you wrote a simple ABAP ALV report you could present the list of possibilities if there are any, or else detect that the list is empty and go straight to creating a new sales order. The code in your current WebRFC should transplant into an ALV quite nicely, I would guess?
Does that sound like it would work?
Steve.