PI 7.31 AEX with ABAP consumer proxy - set sender party

I hope somebody will point out that I'm missing something obvious. As part of my interface setup I try to group interfaces logically by assigning proper parties to them. However, I'm unable to set the sending party for an ABAP consumer proxy that sends a message to a PI 7.31 AEX (Java-only stack).

I researched the following:

• Use parameters in the URL that is defined in the RFC destination for connecting and sending messages to PI: I'm using the URL as described in the online help Connecting the ABAP Backend to Advanced Adapter Engine. Manipulating the URL by adding the other parameters for specifying sender party etc. (see for example OSS note 856597 - FAQ: XI 3.0 / PI 7.0/7.1/7.3 SOAP Adapter) has no effect. I suspect that the parameter ximessage=true essentially means take the header data from the message and ignore any other parameters specified.
• Modify the message via ABAP code prior to sending: Unless I'm missing something in the documentation (e.g. see Creating and Configuring Service Providers and Consumers - Protocols), there is no way to set sender party. I can see that I could set the receivers, but any sender attributes seem only available for read-only access.

Can anybody enlighten me how it's done?

Thanks, harald