If you snap a few caps before loading and if you ram the load with the nipple uncovered and if there isn't excess oil on the flash channel holes walls, you should not have a problem with a mis-fire. The powder should find its own way thru the flash channel and end up under the nipple when you ram the ball down the bore.
It should be noted there are more than a few "ifs" there.
If I were hunting and the first shot had to count, I can see where putting a small amount of loose powder under the nipple before loading would be good insurance. I would not put in enough to create a compacted powder plug though.
I don't think I would use super fine powder like 4Fg though because as the Flinters can tell you, that super fine powder seems to gather moisture from the air when it is humid. If that happened, it would be worse than not putting powder under the nipple.
As for the fuse effect, I have never seen this happen with a Caplock. I have seen it happen with a Flinter but with a Flintlock, there is no concentrated flame like a precussion cap delivers. I might point out that in a Flintlock, the flash channel is open so when excess powder in the channel burns, it generates no pressure. That is why it burns thru to the main charge so slowly.
In a precussion gun, the pressure created by the powder under the nipple is in a confined space and it creates a great deal of pressure immediately. This pressure helps ignite the main charge almost instantly.
:imo: