We have just undergone exactly this. I couldn't find anything that had the right amount of flexibility for
SEO purposes where we felt we needed to keep total control.
We now have an SQL database which has a CMS for all our clients, populated from enquiry/contact forms on all the websites.
We wanted to be able to manage similar websites without getting duplicate penalties, so for example we have (I hope this makes sense)
Website 1 with Ski Resort A
Website 1 with Ski Resort B etc
In addition to ski resorts we have countries, ski tours and ski packages.
When building Website 2 we have Ski Resort A modified as we can either add a ski resort from new or use an existing record as a template. This way we can change the meta tags exactly as we please, change any information on for each resort such as descriptive paragraphs but leave what is consistent e.g Trail Map location URLs.
We then associate the new item (ski resort/tour/package) with the correct website, country, resort etc.
We can if we wish then have the same tour appearing across all the websites with the layout and design changing each time. We could do this for the resorts and other pages too but not sure how different we could make them for the SEs if only changing the CSS.
As we add more websites we simply use different layouts and designs through the CSS and very similar templates for the dynamic pages.
Also in there we have a currency converter (actually we input the rates we want to use so can therefore include any spread in the FX) and other features that we now just change once for all our websites which in theory can be limitless.
This should also work across websites on different servers providing the websites can access the DB.