Adding such thing wouldn't be the problem really.
I see your point in this, however if we add this, i'm not gonna make a point in the user profile too, so you get your favourite always first ;)

In the end that would be up to one of the SM's, but I have to say I do agree with your idea :)

I have no idea how easy/hard this would be to implement, but having an option to choose betewen AUNO and AOMainframe for item links across the site would be very nice. Going though I've noticed the majority of the links are to AOMainframe.

I usually prefer AUNO, becuase of it having comments.

They both seem to have the same ID system, so you would set an id for each item in the guides and append whichever database is set as the user preference.

$aodb = [b]http://aomainframe.net/showitem.aspx?AOID=[/b]
$auno = [b]http://auno.org/ao/db.php?id=[/b]
$setdb = ($aodb or $auno)
...
<a href="<?echo $setdb?>204748">Guardian Tank Armor</a>
<a href="<?echo $setdb?>246274">Cure for Baldness</a>

...you get the idea...

I've never used phpBB so I have no idea what adding somthing like this might require.

(Or to take that a step furthur, add a 'AO-Mainframe' frame at the top of the page that would allow you to switch between AUNO and AOMainframe, ala GameTab and Google Images)