I had the same problem... to address this issue I created a Mac OS X kernel extension which maps simple Linux LVM layouts to logical partitions in Mac OS X:
It's available under an open source license (LGPLv2.1). It's not a complete volume manager, obviously, so only single-disk LVM layouts can be expected to work, and only non-snapshot volumes, etc. but should be good enough for most automatically created LVM layouts for the boot drive.
You still have to find a way to mount the ext3 volume though... fuse-ext2 should do the trick.
Edit: More info, including a binary, is available on my blog.