[Doitsu]

Originally Posted by PaulFraser

Thanks. I'd be interested in more information nonetheless (what the format is called etc.)

These files are often referred to as master mobi files or dual mobi files.

Originally Posted by PaulFraser

[...] and whether it might be possible to create a mobi of that type outside the Amazon ecosystem.

You can create these files with Calibre. (You'll need to select Preferences > Output options > Mobi output > Mobi file type: both > Apply before the conversion.)
I stand corrected.