Yes, when I did the initial project I dedicated a section of the DMP report to discussing those constitutional issues. In my reading of the discussion, for example, if you look at the Law Commission of Canada's report, they simply mention that they thought MMP, using those upper provincial regions, would likely be found unconstitutional, but they never gave a definitive answer. However, the reason MMP would likely be found unconstitutional would not apply to DMP.
MMP requires seats that would span multiple provinces in addition to the regions. With DMP we're simply talking about regions that would span multiple provinces, not the seats. From my understanding of the constitutional requirements it dictates how many seats each province must be allocated. It doesn't dictate how those seats are elected. If that's indeed true, then you could use upper provincial regions while the seats would be completely retained by each provincial district.
You could also choose to implement DMP just like MMP or STV by using provincial regions instead. That change is not complicated. The reason I think it's worth looking at the regions that go beyond provincial boundaries is you get into trouble when you try to incorporate the territories and smaller provinces in Atlantic Canada. If we implement reform without those larger regions, we effectively leave the territories with first past the post and we leave most provinces and Atlantic Canada with something pretty close to something like first past the post.
The whole point of this exercise was to bring voter equality and fairness to all Canadians. I think if we're going to do that, we have to look at exploring the idea of having these larger regions. It could be as simple as, let's say the committee decides to choose MMP as the system they're going with. Elections Canada, for example, could do its work to get ready for the next election using this system while you send in a reference case to the Supreme Court to ask that question. If they say yes, then you can go ahead with it and if they say no, then it's just a matter of changing which regions you're going to use.