> (And simply turning it on for myself won't help me without a lot of testing, 
> as the correct behavior depends upon diverse router behaviors.)

This define only sets the default for a sysctl changable parameter.

I have this in /etc/sysctl.conf:

# Do PMTU discovery
net.inet.ip.mtudisc=1

on all my machines for a long time now. No problems so far.

Martin