How to allow 'UniversalXPConnect' privilege dynamically

Hi All,

I'm working on a XUL application that displays remote xul pages from a 
server.

In order to avoid prompting a warning alert to the user,
I'm giving UniversalXPConnect privilege to the default server by setting 
the following prefs on the client-side application:

user_pref("capability.principal.codebase.p1.granted", "UniversalXPConnect");
user_pref("capability.principal.codebase.p1.id", 
"http://defaultServer.com");
user_pref("capability.principal.codebase.p1.subjectName", "");

This is done at installation time by the installer and it works just fine.

Now I also want to write a configuration tool (that it's again a XUL 
application) to edit the server URL. The user is asked to enter the new 
URL in a form and
I want to give UniversalXPConnect privilege to this server as well.

Here comes the issue.
I didn't find a way to allow the privilege and the user is prompted with 
the warning dialog the first time he tries to access the new server

Things I tried :

1) Using the nsIPrincipal interface but I didn't find much documentation 
about it.
Can you provide an example of using it?

2) Editing the prefs file from my script (by using nsIPrefBranch) and 
adding the following lines

user_pref("capability.principal.codebase.p2.granted", "UniversalXPConnect");
user_pref("capability.principal.codebase.p2.id", "http://newServer.com");
user_pref("capability.principal.codebase.p2.subjectName", "");

but the "id" preference gets never written. Do you know why?


Any idea how can this be solved?

Many thanks,

Guido.