This is for kind of static thing, and I did not find way (almost half year ago) to make it dynamic.., per customer, per call
append_rpid_hf() can accept parameters, so you can make it a little dynamic. For example, some UA changes caller name after activating blocking caller-id, you can add "privacy=yes" in the rpid header.
There is another way which is totally dynamic. You can use avpops to load the rpid and add it to the header. Hopefully someone (Elena? :) can come up with some functions to manipulate avp values. That will make it possible to modify rpid on the fly.
Richard