Can't think of a simple way to do it.
I'd try to use
slave ALSA plugin for force the format to 24/32 bit. Test that first directly with ALSA, with Pulseaudio stopped (
aplay -D). Then use the
module-alsa-sink Pulseaudio module to create a Pulseaudio sink for it (
pacmd load-module). You may have to modify the
module-udev-detect call in
/etc/pulse to ignore the old ALSA hardware, if this doesn't happen automatically.
(Very general advice, I'd have to try it out myself for a step-by-step description).
pacmd list-sinks and
pacmd list-sink-inputs have a
sample spec field that designates bitwidth used, but I am not sure how accurate that is given the various conversions at various places involved.