New defences on appeal can happen, but are very rarely accepted.
As for NCR being raised post finding of guilt, a major part of why it is allowed is littered all over this discussion board (see the Vince Li thread). People have such an unwarranted irrational prejudice against mental illness that real defences would likely get ignored by juries if the accused was identified as mentally ill before their deliberations.
This case analyzes it in greater detail if you are interested:
http://www.canlii.org/en/ca/scc/doc/...canlii104.html