I guess I'm misunderstanding what you mean by "what percentage of its user base Apple disenfranchises," then. If you mean "what percentage of Snow Leopard users become ex-OS X users when Snow Leopard support is officially dropped," then I don't see how statistics such as those from NetMarketShare can help.
Unless you're thinking that
1. There
will be an official announcement;
2. It will be made in such a prominent way as to attract immediate notice;
3. Those Snowy users inclined to change OSes because of
an announced policy of non-support, rather than because of the real-world effects of such non-support, will include a significant fraction who change to a non-Mac OS;
4. That fraction will itself include a significant fraction who act immediately, e.g. in time for NetMarketShare to pick up the movement in monthly snapshots;
5. Growth in use of Mavericks will be noticeably less than decline in use of Snowy, making it clear that the latter can't be explained by the former.
If that's how it plays out, I'll eat my hat.
