I cut thousands of wood pen blanks....thousands. There will always be 'loss' of pen blank sized blanks when cutting crotch, twisted grain, or even crosscut....I sort of figure it into how many I want to have available on the website after they have had to time to dry(and then I re-cull them to get rid of defects). Of course, I cull once again to be sure that only good blanks leave.
That said, I do not use anything to keep them from doing what they are going to do. If you only have enough timber(boards) to make a certain number of blanks, and you wish to have 95-100% of the blanks make it to the lathe. You can try sealing the endgrain with wax, anchorseal, or anything like those products to reduce the amount of evaporation through the endgrain.
You can also store the blanks in a 'better location'. Not too hot, not too dry....etc
Scott (have fun, cull them) B