One of our elgg clients came up with this one awhile ago but the suggestion got raised again this morning.
You know on the Dashboard -> Manage -> Export page, there's a link to the hosting page on the wp.org site? What's stopping you from changing that link to your own such page where you provide the affiliate links? Places like hostgator run their own affiliate program or you can join a service like cj.com which lists quite a few of them.
If you get clients who decide that they want to move out on their own, that link is there in front of them. Hopefully they're start there and earn you a few bucks.
For a landing page, I would place it on the main site as a Page but keep it hidden from any tabs or lists or not to confuse new users coming by your site to sign up.
Hope this helps,