Settings > More Settings > Mobile Network > Network mode > set to GSM only. 

Well, my friend. After 2 years, 1 OS Update and only god knows how many thread versions, an S2 can't do this either without going into settings.

Rooted or otherwise. IINM lah.

It'll be geng if I can simply tell my tasker to switch to EDGE while screen is off.

BUT, if my previous experiences with EDGE in Malaysia holds true, you will DEFINITELY drop calls if an EDGE data connection is active when somebody is calling you.

I had that issue with my old N900 AND my old iPhone 4. Has to do with the data switching implementation of EDGE locally (at least with Celcom) - it gives more priority to data over voice. Not ALL the time, but MOST of the time.

The S3 does this too to my experience. Active EDGE connection --> incoming calls get sent straight to voicemail.

PLUS EDGE gives my shittily shielded speakers the dreaded TENET TENET TENET sound, which pisses the hell out of me.

SOOOOOO,  to answer your question, no. There hasn't been an easy way to toggle 3G/EDGE since GingerBread IINM, for any Android device.

Instead, get Juice Defender or Green Power and get power savings sans dropped calls. You will still get periodic data polling too, so your emails and whatsapps will come in (albeit somewhat slower).