You are right when you say your little tidbit is little known!

My little suite did use the file= qualifier to make an additional copy
of the backup files on a removable diskette for offsite storage
purposes.  I allowed the system to backup its volhist and devconfig
whenever it felt like it but it still backs them up within the
filesystems managed by ADSM.  In the event of a total failure it is
necessary to be able to restore the ADSM db and these two files to
restore the filesystem containing the default backup copies (I
think).

Murray