To directly answer the OP: I know of no way to do that; if you really, seriously, are determined, I bet you could hash something up with dbus-scripts, watching for the screen to dim.

But I'm with GA et al.; you don't need that, you need to find out what's slaughtering your battery.

Candidates to check:

• metalayer-crawler: automatically background crawling on this class of device is just plain wrong, imho. It impairs battery and usability (even when it is working right) if you put in a card with several GiB of stuff; it's known to go berserk when it hits a corrupted filesystem. The built-in media player is pathetic anyway, so nuke this.
• Web browser: sometimes goes mad after going to certain pages (likely Flash-related, but not sure), and sucks 100% CPU for the rest of the session, even when on a clean page. Use a CPU monitor in the statusbar, and close the browser at your earliest convenience after it goes mad.
• RSS reader: I don't use it, but it's widely considered bad for CPU usage.
• Skype: only saw it go nuts once in several months, but you may as well close it if you're not online.
• WiFi searching: scanning for APs is very power-consuming; if you're in marginal coverage, it may drop, re-scan in 10 minutes, and repeat. Turn off auto-connect, if it spends much time in marginal areas; otherwise it's normally not a problem. (But you could anyway...