I have an unmodded '02 3-series that when warmed-up, gets 34 to 39 mpg commuting to work. For more than a year though, I've been taking a train and drive only to and from a nearby train station: it never warms up any more. Mileage has dropped to 24 - 28, so I'd like to add an engine block heater but I can't seem to find the right one.
What I've considered:
1. I don't think I can use the freeze plug type in the aluminum block (does it even have freeze plugs?).
2. I bought an inline "circulating" type and found out I don't have room to fit it in the engine compartment.
BTW, it doesn't really have a circulating pump in it. It works only by convection.
3. At one time, there was an OEM BMW part that bolted directly to the side of the block, but I think it was made out of
unobtanium. 
I'd buy one if I could find it.
Click to see article.
The part# Googles up with 2 hits (well 3 but one is a dupe). In both cases, it seems the part is discontinued or no longer available. It was made and sold in Canada but apparently not in the U.S.
Maybe some of the ecomodders from Canada might be able to assist me here. If you know of a BMW dealer up north that deals in this kind of thing (I've read maybe Saskatoon), let me know.
4. The pad-type that attaches to the bottom or side of the oil pan might work. The magnetic type is out because the pan is, of course, aluminum. There is an adhesive type that might have promise (similar to this one)
...if I could find a flat spot on the pan:
5. Lastly, I may try a "non-circulating" inline type that connects to the lower (not upper) radiator hose.
I'm not sure it will help if the thermostat is closed: won't it only heat the fluid in the hose and leave the long straight-6 block in the cold? I'm willing to try it if nothing else pans out (no pun intended).