Do what you can without changing the size of the slot.... adding resin will decrease overall size, if you don't remove some material first. You don't want to end up with a "most excellent" repair, only to find out your daggerboard will no longer fit down the slot ;-(
I think most repairs are at either end of the slot, so you might be able to get a Dremel type tool in there to lightly grind the area that leaks...(with an extended bit....or custom make something to sand??) and then repair, using decent repair procedures.
You need to find which part of trunk and address that area. File out cracked bits as needed and fill with thickened epoxy, maybe a little chopped glass or cloth patch as needed. No need to coat the entire trunk.