You should be able to support all the way back to IE6 and Blackberry, although you need to include a few polyfills ( http://lhorie.github.io/mithril/tools.html )
Re: TodoMVC - there was someone working on an implementation here: https://github.com/jpmonette/todomvc-mithril
I think it's not fully implemented yet, but you should be able to see the general look and feel of the code.
Also, have you considered adding an entry to ToDoMVC project [1] for ease of comparison to other frameworks? I know it's not an exciting endeavour but it does make comparison much easier.
Thank you for creating it and your time in advance.
[1] https://github.com/tastejs/todomvc