I started putting my alcohol based finish personal recipe on some kanna to see if it could work since a lot of Japanese carpenters put some form of varnishon theirs too. It does make wood very pretty but some finishes really make it much more stable, meaning the dai will move less and any fine setup will las much longer.
Some considerations of my personal experience:
1-shellac alone is not humidity proof enough for a good stabilization.
2-if you prepare a big bottle of lacquer and put the dai inside, when it dries it might crack the dai and deform it heavily.
It will also make it difficult to repair. So I highly recommend to apply with a pad or brush, depending on how pretty you want it to be.
3-some people use thin CA glue in the end grain, I tried that but it won't make a dai as stable as using lacquer or CA glue and lacquer.
4-i believe some thin poly or nitro finishes can work well too, but haven't tried them.
5-protect with tape every part that will be in contact with the blade, so that varnish won't touch them and make them impossible to glue eventually a thin paper or veneer for future adjustments.
Hope that helps.
This was an experiment I did recently. I used some european beech to make a dai for a blade I bought some time ago. It was pretty cheap and was rusty and I had to fix a few things. I found out that beech works extremely well, this is not the longest lasting blade, but it's more than sharp enough to take really thin shavings and a beautiful finish.
I didn't spray water on the wood and didn't apply any product.
To me, this is proof that if you work hard enough on the setup, you can get really nice results. Of course, I love my really good blades and they make this a much easier process for my daily work. But this blade will have a good life too and be very useful in the workshop.