Answering my own question after much ferreting around. It appears that the APPLE makernotes file is a BLIST, a binary version of a property list (plist), and MATLAB's imagesci.rjpg8cimf, invoked from imfinfo, must just throw up its hands and encode the makerfile as a bunch of integers and return a doubles vector.
The EXIFTools source code in PERL for decoding the BLIST is at
https://github.com/exiftool/exiftool/blob/master/lib/Image/ExifTool/Apple.pm
and references
https://medium.com/@karaiskc/understanding-apples-binary-property-list-format-281e6da00dbd
A python decoder is at
https://docs.python.org/3/library/plistlib.html
Discussion: https://stackoverflow.com/questions/27509319/decoding-data-from-plist-file
It would be great if imfinfo were as complete as EXIFinfo. Not only does imfinfo not decode the MAKERNOTES, but it skips over quite a lot of other information.
It is possible to download and install EXIFinfo for various OSs from
https://exiftool.sourceforge.net/
and presumably to invoke it on a file programmatically from MATLAB but that makes its use out of the range of the novice user that I am writing code for, someone who never goes under the hood.
FYI, I've found a few free EXIFinfo servers including
https://www.metadata2go.com/view-metadata
https://jimpl.com/
and one with API access
https://www.conversion-tool.com/api-access/
but that API requires you put your image on a server someplace so you can't just upload a file via a MATLAB RESTFUL call. Not all of these services produce information identical to EXIFTools but they do return APPLE makernotes iPhone acceleration of gravity vector (from the built-in linear accelerometer chip sensor. This data and the bearing from the 3-axis magnetometer (compass) tell you the orientation of the camera locally and given the GPS info the orientation in celestial coordinates, which is what I'm after.
