I'll toss out a different idea here to consider.  I suggest a new
ioctl(2) called FIOVIOLATELAYERING (or FIOBYPASS, or whatever, if you
prefer), which takes as an argument an integer, representing the
number of layers to `bypass'.  It would default to 0 when the file
descriptor is opened, of course, and setting it to some `maximum'
number (and just for the sake of consistency, explicitly preventing
file systems from being stacked higher than that) would `bypass' all
layers.

This would give the flexibility I want, and would be done through an
ioctl(2) as well.