Sounds like a fun and ambitious project 
Our autocomplete engine is really nothing magical and we can’t prioritise creating a service out of it right now. Roughly what we do is parse any require’d modules (using antlr, I think the example Lua grammar will be fine) building a namespace/symbol table and then augmenting that with our built in functions.
Current info on our built in functions is published with every build. For info on how to fetch and parse it, have a look at @britzl dash-docset project https://github.com/britzl/Dash-User-Contributions/tree/master/docsets/Defold - in particular createdocset.py. @Lukas_Palmer made some notes about using Atom with Defold a while ago Big List of Lua Resources!, that might help.