For spells like:

http://www.wowhead.com/spell=145205/efflorescence
http://www.wowhead.com/spell=114158/lights-hammer

There might be others, I don't know.
I've also got no idea how you'd track this, for Light's Hammer at least you can track the cooldown so you know when you can cast it again. There's just no way of knowing who's standing in it.
For Efflorescence it doesn't have a cooldown or provide a buff but lasts 30 seconds, once again we can't track who's standing in it.

@Cluuey You may want to try this again and see if #2 allows this to work.

I've got around to testing this while I try and finish the druid config, 1319 lines and counting.

Still not showing although I'm not sure what I should be trying to get it to show.
I'm also not sure what is giving my that icon which lets me get the mouseover tooltip either, it might be standard I guess. I have noticed that it doesn't include the SpellID which I've got set in the main EH file.
So, I'm lost.

Interestingly that icon shows up at the bottom left of my characters feet as long as I have a target, can be friendly, hostile, a critter, anything, as long as I have Efflorescence down.
I don't have to be standing in it, just in range of it. Moving out of range of it caused the mushroom to despawn.

Just noticed this icon comes up at my feet when I have any target selected and I'm standing in Efflorescence. That indicates to me there must be some way of detecting it.

Ignore the numbers under the tooltip, they're from my HUD.