The information you seek is located at http://www.pikespeakmarathon.org/regstats/2012/ppm_qtime.htm
In direct answer to your question, yes, a 5 hour marathoner has a surprisingly good shot at finishing the PPM. No better or worse than the vets with similar finishing times.
As for the qualifications, most road marathons are more or less the same in terms of difficulty level. Thus they can apply a blanket qualification time that will be more or less equivalent for 95% of them out there.
The same cannot be said about 50k's. Most are on trails and they have wildly different degrees of difficulty. You would almost need a custom qual time for each one out there, a total nightmare. Why not just do a "26.2+" race distance? The issue there is that the 5 hour guy on a 50k trail race is more than likely capable of a 4 hour marathoner on the road. Thus they would get seeded into field 2 and not field 1. There is a reason why the race is split into two fields with the size they are, and one of the main ones is to control the crowding at the turnaround.