I have an idea for using a "full grid" system that would have the whole mud in squares with non-enterable rooms being so because of obstructions which attempts can be made to climb up/down/fly/jump/etc. to overcome. Certainly not a new idea, but one I would like to attempt to work with.

Do any existing codebases make use of both coordinates and room based models? Preferably x/y/z styled.

Is this something I am going to have to code from scratch?

You mean the world is layed out like a grid, with a room at every x/y/z position? That's how the majority of wilderness/overland systems work - there are very few which use true roomless coordinates.

The mud im currently developing works on that principle.  I have a large grid where I can place rooms (and a system to let me place rooms at x,y,z coordinates off of the normal grid and link rooms to them).  Unfortunately my code will probably not be released to the public, but im sure other muds use similar systems.

I am developing a mud that uses both coordinates and rooms - though not in the same fashion as what I understand you to mean. My mud will have rooms the same as many of the older style of mud where they are containers that can fall anywhere perse. And then each room will have it's own "coordinates" inside (more like a grid system inside). For me, this will allow portals to a specific "spot" within the room, blocked doorways/passages, etc.

If you mean your entire world is set up in a true x/y/z coordinate-based grid, and the rooms fall on those - as KaVir said most wilderness systems use that. I found many before when I was doing the research for my game. They are not hard at all to implement (well, the bounding system - movement and affect area - was hard for me, but it was probably just me).

World of the Dead uses a system which you are describing, and is SMAUG-based. As KaVir mentioned, it is not extremely difficult to do - though careful planning and designing can mean the difference between a bad interface and a cool one. My best suggestion is to use SMAUG(if you want something already existing and easy-to-use) and just take careful handling of your interface and the appearance of your grid/map. PM me if you'd like to take a look at World of the Dead's.