Read an interesting solution to this in Roger Barnes Cruising book about drilling a 10mm hole in the aft end of the center board case and sealing it with a bung to stop water splashing in.
Then if board becomes jammed insert a long bolt / metal rod etc and hit the other end with something heavy (mallet / anchor / spare crew member 🙂 to force it down.
Not tried but planning to add a suitable hole as there are several nice but pebbly beaches in my cruising areas (e.g. Blackpool Sands near Dartmouth in South Devon) where I have heard of people having this problem after a lunch stop.
Will let you know how I get on.