I guess to fill below you could use tall thin sprites ? (maybe fading from blue to black ?)
probably overkill on the number of sprites - I don't think it would bother a desktop but might bring a tablet to it's knees ?
I like the sound of using a gl shader but , unfortunately, know nothing about them <img src="smileys/smiley9.gif" border="0" align="middle" />
I am interested to to see how this goes - good luck.