Looking at using a MLD spirit quilt or similar as an overbag with a 30 degree down bag and was hoping for some advice. Does a fully adjustable footbox mean you don't have to size up the quilt to avoid compressing the loft of your inner bag? How does one estimate a combined temperature rating for a two bag system? Cheers.

What combined temperature rating are you looking for?
To estimate the combined temp rating, read the following two threads of Richard Nisley's work:
http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/9378/index.html

http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=18950

[Note: I don't believe the following paragraph, but perhaps someone will point out an error.]
Reading the chart at the beginning of the first thread, it looks like a true 30 degree bag needs to have a clo of 6.5. Then you have to figure out the clo of the overbag/overquilt. If it's also rated at 30 degrees, or 6.5 clo, then the total will be 13 clo. Extrapolating the graph, that would make the rating about -30F. This seems ridiculously optimistic to me. It would be hard to seal leaks with an overquilt; I think it would have to be a pretty large overquilt.

Some people use a combination approach for New England winter camping with a large synthetic outer bag and a down inner bag. Compressing loft can be a problem. Usually this ends up heavier than a single adequately rated down bag. Getting from +30F to +10F will be a lot easier than getting from +30F to -20F.

A 30F degree bag has an average EN13537 LLimit clo value of ~5.88 which equals the ~.91 m2K/W standard stipulation.