Peak meters - overview
I was initially going to add support for calculating peaks by recording the maximum amplitude of each sample written, but as netchild pointed out, such an operation is significantly expensive and generally is undesirable. On top of that, re-reading the 4Front docs leads us to believe that this was intended for hardware with peak meters, as there is a clause saying that the ioctls aren't supported by all hardware, so this requires a slight change in approach.
Approach
First, something like an extension to the channel kobject class (pcm/channel_if.m) seems to be in order. From the 4Front docs, "It is not supported by all devices. Error (errno==EINVAL) will be returned when this information is not available." With that in mind, a default function (called when hw driver doesn't override) can either return this error or optionally process this in-kernel (controlled via sysctl?).