Are there any code restrictions involving in-wall convection ovens?
A stand alone appliance will always come under Underwriters Laboratories jurisdiction. The manufacturer's install instruction will dictate proper guidelines. If a local inspector is involved he can and sometimes does ask for this literature and check things out. Involving a convection oven, I would rigidly follow these instructions and even might error on the side of caution.
The code that applies depends on where you live. In California, it is the Cal Mechanical Code (CCR, Title 24, Part 4), which often incorporates other codes (plumbing, electrical, etc.) and standards (NFPA, etc.) by reference. Your local code enforcement authority may also have requirements specific to your locale (also a great place to inquire as to what codes apply in your area/region).