No, you cannot boot the BeOS off of a logical/extended partition, so we don't let you install into it either. At this time, we're not sure if this is a feature we'll support in the future.
We only know of two operating systems which can boot from a logical partition, Linux and OS/2, so it's clear that an OS (e.g., Windows) can be a commercial success without this feature, and that supporting it, <ahem>, does not guarantee similar success (witness OS/2).
Still, Linux proves that you can support it and be successful. We're seeking to emulate that example, though no commitment for which if any release will support this feature.