Has anyone explored the option of a (custom?) SA rule that increases score the more times a message has been submitted to a hashing database?
You mean like the RAZOR_CF_* rules, which alter the score based on how high a confidence factor razor reports? AFAIK SA has had this feature since 2.50.
Admittedly it's not quite the same as the number of reports, but IMO CF is significantly better than just the number of reports.
It's also not a mathematic expression, it's instead divided up into block ranges so that they can be GAed properly. Trying to apply a linear or other equation based score increase isn't generally a good idea anyway. Email is based on human behavior, something which is inherently weird and complex, and better done with GAs than equations. Take a look at the BAYES_* scores as an example. They are not a continuously increasing function, and that's not a problem, nor even surprising.
