Sure can. You can pretty much use anything that gives out colour as a lightsource when you use GI.
Just give the surface a very high luminosity value(10, 100, 1000, 10,000, depending on how bright you want it) and un-check 'visible to camera'. The glow from the object will be picked up by the envirolight if you use GI at render time. Higher GI settings give nicer results, a relative detail of 1 will likely result in a very blotchy lighting.
You could also use a cloud layer with extremely high colour values in the colour or ambient tab. Un-checking 'enable primary' will stop the rendering of the actual cloud but leaves you with it's lighting effect on the environment around it. You can localise the cloud to limit the effect and drag it around or, leave it filling the whole sky for a global lighting.
