I work at Zipline Games and we're building out a server side hosted lua system. We're targeting it at game developers, but there is nothing game specific about the lua handlers you can write. You can find out more about it at http://getmoai.com
I just want to say: thank you for MOAI. It is an awesome way to get an app done for Android/iOS platforms simultaneously, and I really look forward to the future of this framework. Without question, it has been a very enjoyable experience - develop on Mac OSX, simultaneously deploy on both Android and iOS, and then ship. Really, a pure joy to use.
As well as Mongrel and Tir, there is LuaNode https://github.com/ignacio/LuaNode/ which is modelled on nodejs. I think there is another node variant too. Also, with a slightly different feel is the Lua Nginx module which is also async with coroutines, but using nginx to provide all the IO. https://github.com/chaoslawful/lua-nginx-module
If you don't want a hosted solution you can look into mongrel2 and tir http://tir.mongrel2.org/