On Tue, Feb 25, 2014 at 10:09:16AM -0500, Paul Davis wrote:

> the option doesn't alter what the JACK backend sees from ALSA (or
> wherever). it changes which channels get represented by JACK ports.

Wrong. If things were as you claim, this should just work:
(on a 64-ch card)

fons@hypatia:~> jackd -P70 -p256 -t2000 -u -dalsa -dhw:0,0 -r48000 -p256 -n2 -i16 -o16
...
ALSA: cannot set channel count to 16 for capture
ALSA: cannot configure capture channel
cannot load driver module alsa

The -i and -o values ARE used as parameters to ALSA, and
not just to determine how many of the available channels
are mapped to Jack ports. You can also verify this in the
sources.