How do I use RSWUWFML2s 'Preparation Exit'

[Mike Pokraka]

G'Day folks,

Version 6.20 - how can I add custom text symbols to RSWUWFML2's SapScript
text that goes into the email?

I did some research, but the cryptologists at SAP have me a bit baffled on
this one:
A user exit is provided, but lacking doco I looked on OSS where I found
the answer in note 808971:

"The first user exit ... to enable the user to define more variables that
can be used in the SE61 texts."

Bingo, exactly what I want. So how do I use it? According to the note

"The entered function modules must fill a certain interface. In the case
of the "FM for preparation phase", this is an interface without any
parameters."

which is about as useful as explaining how to peel a potato. I sifted
through some of the code and ... well so it calls a FM without params near
the beginning of the report. So I assume something needs to be
declared/stored globally/something'd.... but what?

Any input appreciated from anyone who's gotten this to work.
Thanks,
Mike

(cross-posted on WUG and SDN)