Peter Norvig

Get the 500 f4 used, and if you don't want to keep it, sell it for about the same price after the trip.-PeterKenya with 500 f4 GalleryKenya with 100-400 GalleryWith 500mm:With 100-400:

I think you want to get out to somewhere in the 1000mm-e to 1500mm-e range.  Here's one at 1000mm, with an APS-C camera, not m4/3 (but the same lens on a m4/3 with an adapter would probably do...

Also in the gallery: http://pn.smugmug.com/Nature/Lunar-Eclipse-2010

How about a 75-150 f/2 (77mm thread) with a built in, switchable 2x extender (like the 1.4x in the Canon 200-400).  That would take you out to 600mm-e at f/4.

I agree that the 35-100 is the best choice for one lens.  But if you can have three lenses, then the 20, 45, and 75 give you ample FOV coverage and 1.3 stops more light.