Hi Alex
Standard Solution is to have separate payment method. Am sure you know this
If you want one payment method Vs Many DME Trees, User Exit is your only Option
But I can offer you a workaround, I have myself done this. The world's favorite phone brand does it this way.. I cant take names here 
Have a Single DMEE Structure. In that you can add as many nodes as you want. Use the Condition Tab to make it applicable only to a Certain Business Scenario
For Eg: you want to populate CXD in a Node when House Bank = X and BXD when House Bank = Y. In this case, add two nodes with Constants CXD and BXD. And put a Condition that 1st one is applicable when HBKID = X
This way you satisfy SAP's requirement (One DME Tree per Payment Method) and yet you get what you want
Ajay M