I did end up making a little utility: BD Region/Country code checker
It's slightly less useful than an earlier prototype I had, but I realised you could, with some effort, say it was "circumventing geo-restrictions", which is legally a bit grey.
Anyway, this version also does it's job, but only lists against which values the region and country codes are being checked. These lists can be whitelists or blacklists (or something in between). So, for example, if it says region A, it might be a region A disc or an B|C disc.
Here are some examples:
It appears that Sentai has changed it from checking that the country code is either US or CA to checking that it's not JP. That's probably the reason why those earlier titles don't work on certain devices.
I decided to test this with the High School of the Dead Limited Edition set (as the discs were re-authored to include the OVA), and I got this:
This set was released in November 2014, just before the Geolocking began. I don't think it's an error, more so Sentai probably trying to create a workaround on the set or they left it region free since the LE price was high.
In other words, I'll keep note of your method for future use!