A few years ago, I used to help organize the Galveston County swap (before meb_lady and Margie made it a smashing success).
I've built a new web site which I wish I had back then. It was designed to facilitate the organizing of plant swaps for both organizers and attendees:
You are invited to join the forums (it's completely free) and post swap notifications in the event calendar.
Here is a link that might be useful: http://www.plantswap.net/