IDK, I guess the most we'll see is a small shift of people from port jeuno to Ru'lude gardens with the /yell feature...

If it's not possible (due to mechanics) to extend a shout across a large enough area selectively, then would it be possible to have a PC (or NPC if required) that listens to shouts, and transmits that information to 3-4 'gossip' NPCs who then repeat what was transmitted to them in /say (or shout)?

Example:

1 NPC that listens located midway in port jeuno
3-4 NPCs capable of /say; 1 located in each respective port (or whitegate)

Playercharacter Bob yell/shouts '{Ballista} {Do you need it?}'
Listening NPC hears Bobs yell/shout and sends that message (along with who shouted it) to the appropriate Gossip NPC in San d'Oria.
Gossip NPC Loudmouthlarry repeats the message in /say '{Ballista} {Do you need it?} - Bob

You could probably say something along the lines that Loudmouthlarry has a linkpearl and is getting fed the latest gossip from town... if you clicked him, he'd tell you to hush down as he doesn't want to miss anything.