“anchored” channel entry placement

My client wants within his blog to have all entries order naturally by date, with the exception of certain “special” entries. These “special” entries will have a specific placement… e.g. he wants the fourth, seventh and eleventh entries to always remain “anchored” in the same order placement. So when new entries are entered, everything shifts down making room for the new entry, except these “special” entries.

These “special” entries need to have the same content management as the “normals” also.

I hope this makes sense. Among the many requests my client has, I’ve been able to figure out all but this one. Any suggestions on how to accomplish this?

Although this is not the reason for the request, I can see a practical use for this. It would allow someone to place and keep advertisements in the same position a blog without them having to move them whenever an entry or comment is made. (always trying to think of a way to capitalize off ideas )

Thanks, John