@Dasj I don't know what the go to strategy is for "seeding" chatrooms. Part of the issue is IRC is the de facto chat place for most of the Drupal community.
Another issue is Drupal is pretty niche compared to PHP development in general; SO & other SE sites are much higher volume. Hopefully, D8's embrace of the wider PHP dev tools will eventually spur growth.
But, as is, this isn't a bad place for off topic chat, especially where there's much back-and-forth debugging.