World: r3wp

What you would be developing is already in R3, so It's a lot simpler 
to just interface R2 and R3. Probably the most straightforward option 
for that is 0MQ

I mean touching as in changing anything in R2, such as including 
an extensions interface

Why is that not an option? Maybe not very likely, but if enough ask 
for it, maybe then?

Good luck getting Carl to respond to anything REBOL related

Ah yes, that's a good point.

If you're approaching it from a theoretical standpoint, then yes, 
this whole situation could have been avoided by doing things such 
as implementing the new extensions interface in R2 first

R2 is in the backwards-compatibility phase of its existence. No new 
changes are being made to it that would break a lot of R2 code.

That being said, something like this might be possible to add on 
to R2 as a library, though I haven't given much thought as to how.

To integrate the extension interface into R2 would be a huge task, 
and I'm not quite sure how to get around the lack of extendable objects 
(to build the module system on, since extensions are integrated with 
the module system). Adding the handle! type would be easy; adding 
the command! type would be a bit harder.

It would be rather difficult to fake the command! type if you are 
doing the library method, but you could use a similar method to that 
used to fake the closure! type. I am not sure how much you could 
replicate the value marshalling, and callbacks would likely be impossible.

Have you looked into what would be necessary to make a host DLL wrapper 
for R3 that could be loadable with LOAD/library? Is it possible to 
make a host with LOAD/library directly?

These are the sorts of workarounds that created the software bloat 
of the world

To be more concrete: I mean a lib interface that uses the INIT, CALL, 
QUIT structure on the C side. Uses the access to parameters via frames 
and same return concept. And callbacks like done in R3.

As said, callbacks are most likely to be impossible

Which would diminsh the usefullness of such a project