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/)
@Jedrek: "What is a difference between Re-JIT and reflection?"
Reflection is used typically by application code to inspect or emit new application code. While Reflection can be used to create new, dynamic code, it cannot be used to dynamically modify statically-generated code (at least, not in any way I'm familiar with that is comparable with what ReJIT can do).
ReJIT (and Profiling API-based IL rewriting in general) is used by a separate diagnostics tool (that the application designer had no knowledge of, and that has no knowledge of the application code), to slip in its own instrumented copy of functions that get called whenever the application designer thought she was calling her own code.
A few questions came my way, which I'll answer here:
"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.
"Is ReJIT in the 4.5 Developer Preview?" Yes! Go for it!
"Where can I get a copy of that web site demo you showed?" Nowhere, yet. Cleaning up and releasing the demo, with some of its sample code, is something we will consider for later. But that would take some time, as it's currently heavily tied in with other ReJIT testing code, and split among several different components.