You could try using the alsa plug interface which can do format conversions (including interleaving and de-interleaving), but I seem to remember that the current pd alsa code does not let you use generic alsa device names but just card numbers which correspond to the hardware interface (I may be wrong here).
Fortunately you're wrong at last:
Good to be wrong! :-)
the latest Pd from Miller's site, pd-0.37-1test6, supports setting the alsa device as a string again by using the new "-alsaadd" option.
Sounds great... I have to upgrade to the latest and greatest... -- Fernando