If it's multithreading you're looking for, Lua has a built-in Coroutine lib that's supposedly available in the WoW API. See
http://www.lua.org/manual/5.1/manual.html#5.2. However, Coroutines are only a psudo-multithread process. No other code can be run the same time a Coroutine is running.
Widget inheritence can already be done in the Widget API that WoW uses. This is done by creating a virtual frame in XML and using the inherits attribute when creating a copy of it in XML or passing its name in the
inheritFrame argument of a
CreateFrame() call.