Yeah, from what I understand, this is one of the main intents of the Lua engine. It's still very much under development, but making Lua capable of doing everything the current debugger interface is capable of at a low level may very well be its goal (hopefully someone who spearheaded the Lua integration stuff can confirm or deny here).
In my somewhat limited opinion, the more we are capable of interfacing MAME's debugger with debuggers that have been developed for years (GDB, Ida Pro, etc), the more developers will feel comfortable working with MAME. I don't know how possible this is or to what extent we can do it though, so I may be off base here
I've never spent the time to really understand the various ways the current, internal debugger pokes into the MAME execution loop, but I'm really interested in assisting with the work to make it more accessible. There was some talk about developing a general API layer, but I believe real life got in the way of a final draft of that spec.