I personally can't think having oil "too" cool is a bad thing in a normal riding situation. In extreme cold, well, sure, but few of us would ride in zero degrees much. When you do, just make a cheap cover out of some vinyl or naugahyde and velcro it over the cooler.
Why not consider a Harley style thermostat/cooler? Back in the '90's Harley's kits came with an automatic thermostat/bypass that went in between the cooler and pump. Flows a little when cold, opens fully when it got to 180ish or so. Used to be able to buy them seperate, but I can't find them quickly when searching.
A company called Derale makes 'em, too. There's more styles available, but this gives you an idea:
http://www.northernautoparts.com/ProductDetail.cfm?ProductId=3164
Also, do a search for Lockhart brand oil coolers. Both Derale and Lockhart have been around a looong time.
To get an idea of how "big" an oil cooler could be, look at the '86-92 Suzuki GSXR 750 and similar 1100's. The engines appear to be water cooled, but have short heat sink fins on the rocker cover, head and cylinder along with the big honkin' oil cooler in front. The pistons crowns were squirted with a constant flow of oil from underneath. Even that technology wasn't new in 1986: WWII fighter planes used it. Pretty bitchin' idea that worked very well. Durability of those Suzuki engines was and is second to none.
Back in the day, I roadraced and street rode a '90 750 in 30 degree weather with no problems. As somebody mentioned before, just don't expect to run 20W-50 then... Remember, new cars get along just fine with 5W20...