Exiled claret, they already have a system that gives places to runners who have completed a marathon in the last 24 months. The only caveat is that they have completed the previous marathon in under a certain time. Thats the "Good For Age" system.
London is a race that is physically restricted to a certain number of runners (35000 ish?) but has more than 100 000 people who wnat to run it. That means you are always going to have over 65000 unhappy people regardless of the entry method/criteria.
Also if you did a first come/ first serve it would be filled within the first two hours of applications opening, and then the forum would be flooded with complaints of the web site not accepting their entry/crashing or freezing etc etc