How can one can convert a Denso mapcode (e.g.,
549 544*88) into GPS coordinates or a human-readable address?
All the resources I have found so far such as https://japanmapcode.com/en perform the conversion from GPS coordinates to Denso mapcode.
I managed to find a site that does this conversion between numerous different formats, including plus codes.
The site is located here http://saibara.sakura.ne.jp/map/convgeo.cgi
Enter your map code under マップコード (make sure you also select the corresponding radio button for this option) and then click Send. The result will appear under 世界測地系
I note also that Denso have a "Drive! NIPPON" Android app that accepts these codes.
Screenshot from http://saibara.sakura.ne.jp/map/convgeo.cgi: