telescopic keel ?
If I understand what you are describing, it probably would not work for a variety of reasons none the least of which is that the position of the keel fore and aft rarely aligns with the position of the mast. That said, boats woith center boards and dagger boards are not that unusual.
To clarify the terms: Centerboards are appendages that can be raised and lowered on or near the centerline of the boat. They can rotate up into a trunk or rotate below the boat. Daggerboards are a type of centerboard that raises vertically or near vertically in a trunk. Swing keels are a type of rotating centerboard that actually contains a substantial portion of the boat''s ballast. They may be housed in a trunk like a Tartan 27 or 34 or hung below the boat like a Catalina 22. In the case of the Tartan 27 or 34 type, they are more frequently referred to as a Keel/ Centerboard (abbreviated k/cb)
A drop keel is a daggerboard that actually contains a substantial portion of the boat''s ballast. These are easier to lock down but can be more easily damaged in a grounding. They generally have better shape than a swing keel and can be more robust, but not always are. One great advantage of a daggerboard is that you can have a large lead bulb that when fully down gives enormous stability and when raised still is more effectve than trying to ballast a full length shallow keel.
While the basic idea of a retractable keel has been around for a long time, few manufacturers build them today as they are more expensive to build and maintain, and, except for the case of a dagger board with a bulb, do not offer as good performance as a fin keel.
Keep thinking-As a person who would like to see dagger boards with bulbs become more common and has designed one which used a stainless steel tube frame that extended becyon the top of the board to take the moment loads- I''ll be glad to kick ideas around you.