I was watching a YouTube video last week of a high end furniture producer in the US; he was using some quite substantial walnut stock to mill to a size that would eventually become a barn door construction made of a number of finished boards “dominoed” together.
One of the boards he wanted to use had a very visible split in the end grain that would affect the finished size, so he wanted to address that so it wouldn’t get more damaged through his thicknesser and sanding machinery. The split was of a size that you would not have been able to get a brush or cocktail stick into it to apply glue to the surfaces.
He poured some glue across the top surface of the crack, here comes the ingenious bit, he put his workshop vacuum nozzle under the crack and used that to suck the glue into the split and then applied some clamps after he had cleaned the surface glue that was left. When the clamps came off the split was entirely stable with no visible trace of the crack that had been there before.
I thought the “fix” was worthy of sharing to other makers in case it is useful for your projects. It was fascinating to watch it work in the video - I’d never seen it before and certainly would not have thought of doing it.