Email address for invoice recipients in sales order

Dear Gurus,

we have following requirement:

We want to have a separate field in customer record for email address of invoice recipient. Let's say this field will be KNA1-KATR10.

This field should be copied to sales order, where user is able to change it. When invoice is done later the invoice should be send to this address instead of default address of invoice recipient.

My idea was to copy the email address to address of invoice recipient in sales order. Therefor I wanted to create a new address segment (just as copy of address of invocie recipient, only with changed email). So I tried to use EXIT_SAPLV09A_004, but when I change address number of invoice recipient, it is not displayed on screen(Obviously the address of partner is red again after user exit).

Another option is MV45AFZZ. Here I changed address number in form USEREXIT_SAVE_DOCUMENT_PREPARE. It worked, but this is to late, because user should see the email address.

Has anyone an idea, if I can use another userexit or another idea how to setup the requirement.

Thanks

Andreas