LuaPlus' LUA_REFCOUNT implementation
for Lua 5.1 exists at [1]. I removed it years ago in the next
commit, but it is there for reference.
It shipped in a number of commercial
products using the reference counting implementation [2].
There are discussions within the Lua
mailing list archives about its performance and memory
improvements, but you would have to search them out using 'luarc'.