You can buy a rather large syringe (for horses) at most pet supply stores
and some hardware stores for a couple of bucks. I've used these over the
years to inject Epoxy or woodglue into tight spots. Works great but with
Epoxy it's usually a one use tool. I'd drill the ends to stop the crack from
running as previously suggested and then fill as much of the crack as
possible with Epoxy and clamp it with as many clamps as you can get on it.
Might want to put wax paper under the clamps so they don't become a
permanent part of the daggerboard ;-). Clean up the squeezed out Epoxy with
acetone before it dries.
Charles