Higher Order Functions in VBA, the dirty way
Things I like

This idea.
You've explicitly declared ByVal and ByRef.
All methods are short, clear, and concise.

Things to watch out for

All of these functions take variants in as parameters, by necessity. Client code runs the risk of passing in a sequence that isn't enumerable. These really need a comment and maybe a VB_Description attribute describing what kind of arguments the functions expect.
You also may want to consider handling runtime error 438 &quot;Object doesn't support Method&quot; for the functions that use For Each loops. There's not much you can do other than return a more meaningful error message though.
If you change the loop in Map to a For Each and manually increment i, all of the functions will be compatible with collections as well as arrays.

Nitpicks

Underscores have a special meaning. They indicate event procedures. So I would change initial_value to initialValue.
While I quickly understood that el stood for element, there's no reason not to just spell it out.