Converting large unwieldy single site to multisite

I am part of a team working on a very large WordPress site that has been churning along for several years (very little blog content, mostly pages and custom content types). During this time, a lot of ‘areas’ have been added to the site, each with its own special needs and each with its own look.

So far, we have been managing this, but we are beginning to see that if we had it all to do over, that this would be a good candidate for multisite. Now we’re at the point that we’d very much like to turn this large and complex site into a multisite setup with a number of smaller, more manageable sites. We are seeing certain branches of the page hierarchy as becoming sites of their own within the new multisite setup.

Researching this issue, I find lots of advice on how to turn multisites into a single site, but nothing on going the other way.

A few thoughts:

If we could selectively export portions of the database (e.g. every descendent of a particular page), that would help a lot. Or if we could selectively import at the other end, that would help, too. Does anyone know of a good way to do that?

We would like to keep this as a sub-directory based multisite, as that would keep the client happy, and the hope is that existing URLs would still work. All the how-to’s I’ve seen say this can’t be done, but if pieces of the existing site were brought into a freshly created multisite (see previous paragraph), that could be a workaround.

Recreating everything by hand is not an acceptable solution, as we’re dealing with hundreds of pages and an equal or greater number of custom content nodes.

Thanks in advance for any and all advice and/or suggestions.