I think trying to case harden footpegs is a bit overkill, as stated carbon is the magic ingredient that is needed in steel to make it hardenable, from memory once the carbon content is greater than 0.2 percent it can be hardened.
The greater the carbon content the harder the steel can become, the trade off being the harder it becomes the more brittle it becomes.
Using a steel that can be hardened may make the pegs brittle which would not be desirable for a peg, the best option would be to lay some hard facing rod onto the tips, I have a couple of rods here you can have if you want to go down that path.
If you do you will probably find the peg face will outlive the hole elongation problems of old pegs!
I don’t believe hardening gears after undercutting is the way to go either, it’s my understanding that hardening nearly always distorts the original size and so gears are machined slightly oversize, then heat treated for hardness and then ground to size.
Also I am of the opinion that dogs aren’t that hard and are not meant to be. The gear dogs come into contact quite harshly but only every time you change gears.
Teeth on the other hand come into contact with each other several thousand times a minute.
Worn gear dogs usually have the corners rounded off, teeth usually chip, I believe that if the dogs were truly hard like the teeth then they would most likely chip instead of rounding their corners
