World: r3wp

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