With some of the small lots of stats i've got on the unitab ratings, i've noticed that 100 raters obviously win the most, however there seems to be a dip at 99, a slight increase to 98, it peaks at 97, then starts to diminish again after that.

i.e. for a few months last year i kept the stats on ratings across all venues in races that were class 3 or better. The following are the number of winners that had the specific unitab rating:

Rating - Winners
100 - 483
99 - 135
98 - 173
97 - 219
96 - 244
95 - 190
94 - 187
93 - 153
92 - 130
91 - 121
90 - 79
89 - 54
88 - 37
87 or less - 87

Total Races 2292

As you can see there might be some room for laying 99 and 98 rated horses in the right circumstances? That's all the stats I can really provide but may give you a starting point for some form of system?