Your comments

Correct me if I'm wrong, but isn't implementation of this feature likely to largely be a case of implementing symbol parsing for project files?

As mentioned, there is already an API for autocompletion - it just only displays tooltips for builtins (which IMHO makes it a little bit useless). It is also not relevant to the context of the caret (showing the autocomplete dialog after typing an object or class name still just lists global function names), but I suppose one feature is linked to the other.

If the engine could parse project files and work out the available members, perhaps read method header comments, perhaps even follow dependencies and parse them out too - then perfect.

Anyway, +vote