Hi! Yes Peter, 8051 can do this job. In my previous job I have made frequency meter within range 10Hz...150MHz. It was about 10 years ago so I have used an old 8049. The main problem was input amplifier... Not digital - it was simple, but bandwith of input. Prescaler has been made using ECL logic ICes.
This frequency meter could also make some mathematic operations using input frequency. So, if 8049 can do this job, 8051 can do it too ;). But your project is very interesting to me, so in this moment I am going to take a look. In my opinion your web site is very valuable. Maybe it doesn't contain super graphics but informations are very, very useful.

Jacek