That's what I would worry about too.
May I suggest that you make a tartlette for the experiment. What if lard does make the top soggy and crumbly?
I just looked it up. Whole egg, 100 grams of the edible portion of hard boiled egg, contains 75 grams of fat, so 75% for whole egg. From
Wikipedia - Egg as food
I have never made an egg wash with just egg yoke. Is that something people do? I have used whole egg whisked together, either with or without the addition of some water. I have also used just whisked up egg white (I don't think I ever added water to that.).
The pie crust I make for tourtière is very close to half lard, by weight. Fat and water don't mix well, so I think a more water based wash is less likely to penetrate than a fat.