Ok. Here comes the Inactivate, Mathematica 10's powerful feature.

To replace something inside Hold, explicitly your question, use this

Hold @@ (Inactivate @@ Hold[{1, 2, x}] /. x -> Sequence[3, 4]) (*Hold[1, 2, 3, 4]*) 

And you mentioned you actually want to use this to inject Do iterator in Compile. This can be done directly by Inactivate without Hold stuff. Use this

Activate[Inactivate[Compile[{}, Do[code, iterators]]] /. iterators -> Sequence[{i, 5}, {j, 5}]] 

I personally think that with Inactivate and Activate, we can think many things differently now, especially meta programming.

Ok. Here comes the Inactivate, Mathematica 10's powerful feature.

Inactivate could not solve injecting expression into Hold.

But you mentioned you actually want to use this to inject Do iterator in Compile. This can be done directly by Inactivate without Hold stuff. Use this

Activate[Inactivate[Compile[{}, Do[code, iterators]]] /. iterators -> Sequence[{i, 5}, {j, 5}]] 

I personally think that with Inactivate and Activate, we can think many things differently now, especially meta programming.