I looked for ages for the same thing.
Then I just renamed my ext3 image file to filename.dmg and double clicked it in finder. After a warning it assigned it to /dev/disk3 and mounted it as ext2 under /Volumes. Using diskutil or ext2FSmanager it is possible to unmount it without ejecting it. It remained assigned as disk3.
My system: 10.5 with ext2FS 1.4d4 installed