How would you measure amount of work you want to reward with a coin? In both DNS and torrent examples one can influence a supply of work to be done rather easily
It's a broad idea to construct a secure PoW, with all that's required to make it secure, but to try and make it applicable to areas like number theory. See PrimeCoin for a successful example.
Also your concerns are unfounded: See Namecoin and Primecoin. I'm thinking these two ideas could be combined.
Also your concerns are unfounded: See Namecoin and Primecoin. I'm thinking these two ideas could be combined.