o add a `flags' keyword; values may be schg, sappnd. Extending this list
           to all flags supported by chflags(1) may be considered.

i did this several months ago.
   
   o add a `-i' option (`i'mmutable): Set the flags specified by the flags
   keyword.
   
   o add a `-m' option (`m'utable): Remove the flags specified by the flags
           keyword. This can only be done at securelevel=0.

obviously, mtree(8) will not attempt to enforce that restriction..
   
   o customize /etc/mtree/NetBSD.dist and/or /etc/mtree/special
   
   The `-i' and `-m' options should be inverse operations so that we only need one
   mtree input file.


these are a good idea.  would you do this work?