I’m not aware of any rate limits within Growl that would cause it to ignore notification requests. I assume Growl is implemented as a simple queue, so notifications would be queued up until the application had resources to display them. I know I’ve seen dozens of notifications come up when starting certain apps (iStumbler comes to mind). The more likely explanation is that Dashboard is having trouble running multiple instances of osascript through widget.system. This would be a great question for the Dashboard-Dev mailing list, on which several Apple engineers lurk.

Regarding the icon, yes this should be possible with some editing of the AppleScript. I did the simplest implementation and just used the Dashboard icon, but you should be able to access a file on the filesystem (within your widget) or pass it in as you say. I haven’t done this myself, but it’s all documented on the Growl website.

Good luck! I’m interested to see how your implementation shakes out.

-Andrew