Quick correction to my previous comment:
Can ReJIT be used to turn on enter/leave/tailcall hooks?"
No. A reasonable thing to consider for the future, but it will not be available in .NET 4.5.
While there is no explicit way via the ReJIT API to specify whether the codegen flags should allow or disallow generating calls to the ELT hooks, there is a slightly tricky way you can achieve this, even in the 4.5 developer preview. I plan to write about how to do this in my blog eventually. (http://blogs.msdn.com/b/davbr/)