In particular, Lua-CJSON [1]
[1] http://www.kyne.com.au/~mark/software/lua-json-performance.h...
(I know, everyone wishes their favorite language was represented and not all can be tested. I'm just posting this in case the OP does a "Round 3")
EDIT: fix typo
BTW, LuaRocks is supported [2]
[1] https://github.com/chaoslawful/lua-nginx-module
[2] http://openresty.org/#UsingLuaRocks