I like your thinking however i have found a major problem.

From my database which only does Saturday metro racing over the last 2.5 years.

Total selections > 50-1 = 9272
Strike Rate = 0.64%
Profit (using BF starting price) -44.29%

Looks great!

Now if we decide we want to make a living out of this
I add my filter of "total money matched on horse" > $5,000 (Not really that much money to be traded)

I get the following:
Total selections > 50-1 = 142
Strike Rate = 1.41%
Profit (using BF starting price) -22.54%

142 Horses is just over 1 bet per week and Saturdays has by far the most volume on Betfair so i would assume there wouldn't be many weekday selections either.

Anyway, I like the theory but it looks a little impractical based on my experience.

I am almost finished writing an app which coallates UK race data, perhaps your system may fair better on the UK markets as there is much more liquidity than in Aus. I will check it out when i am finished and have collected a bit of data.

Cheers
Glen