its a nice idea!
perhaps baby steps first though - what would be nice is more understanding/statistical analysis of Fan Survey vs Merchandising/Attendance
This may reveal the components that contribute to the attendance and merch, including the value of home grown players, particularly over time.
maybe there just needs to be another Fan Survey category, which is exclusively for 'home grownness' - similar to your HMI. that would be a good representation of the HMI and I assume it would already exist int he fan survey calculations as a sub-component of something already.