"nethack.alt.org has patched the initial seed calculation, but as nethacker Adeon has demonstrated, it's still feasible to figure out the game's seed by observing random effects."
To me the article reads as though this is more than just an issue with poor seeding. If you can figure out the initial seed by observing events then the PRNG itself needs to be replaced.
The article suggests reseeding the PRNG after x random numbers have been generated and to me that sounds like a fairly good solution. It has certainly be the basis of other PRNGs: http://en.wikipedia.org/wiki/Fortuna_(PRNG)
Absolutely. However often you reseed the generator you need it to be unguessable.
Server activity would be a good source of entropy with the caveat, as you point out, that you need to be sure that there is more activity on the server than just the person receiving the random number. If server activity is the only source you use though you're vulnerable to some attacks. If an attacker can force your server to reboot or can utilise all available connections, locking anyone else out, then they could control all the server activity you're relying on.
The solution is simply to use multiple sources. Server activity plus any other sources of entropy you have available on the server.
To me the article reads as though this is more than just an issue with poor seeding. If you can figure out the initial seed by observing events then the PRNG itself needs to be replaced.
The article suggests reseeding the PRNG after x random numbers have been generated and to me that sounds like a fairly good solution. It has certainly be the basis of other PRNGs: http://en.wikipedia.org/wiki/Fortuna_(PRNG)