Easy User code injection

[Patience via Digitalmars-d]

One problem with many applications is they are static once 
compiled in the following sense:

There are apps that have certain behaviors that may be incomplete 
are undesirable. Usually it could be that the designers used a 
hard coded value for something when it would be better to have 
made that variable a "setting". The problem is, the only way to 
change this is to re-compile the code.

It would be extremely beneficial if the user themselves could 
modify the code. Since they obviously can't recompile without the 
source, i.e. all commercial apps, they have to ask the developers 
for the enhancement or fix.

The developers could write a complex object oriented design and 
allow the user create objects that modify existing objects and so 
forth(a plugin like system) but that creates way more complexity 
than required for usually simple modifications(a few lines of 
code).

Does anyone see how it could be possible to make an efficient way 
in D applications to allow for "user modifications" almost as 
equivalent to having the source?

Code injection/Hooks is probably the way to go about this but one 
has to worry about performance in some cases, the implementation 
details in D, and how to present this to the user in the best 
possible way.

A "scripting" like interface would probably be required but not 
necessary as we could allow the user to create the code somewhere 
else.

Another idea:

Suppose the source code is internally encrypted and when the user 
wants to modify a part of it, the application can recompile that 
part of the code and replace the original. Doesn't seem like a 
great idea but probably only slightly worse than reverse 
engineering the binary itself.