I realized this may not have been completely clear...

      If you put some serious intelligence on the board, you can even have the
      board do scatter/gather DMA directly into mbufs.  I'm a little leery of that,
      since you need to have a piece of "hardware" understand the layout of a 
      software structure, which might change -- [...]

   Mbuf `clusters' use an optional function pointer to a `free' routine,
   so you can DMA to and from the `cluster' area, and not have to teach
   the board about the structure layouts.  This is also advantageous
   because you can make your `cluster' whatever size is convenient.

The idea here is that you can fake creating the `cluster' yourself, by
setting the pointers in the mbuf, and setting the M_EXT bit in the
flags.