I can confirm that using the 11.2 boot loader works -- my co-worker did that with another UFS+raidz2 machine (which was upgraded to escape the silent hangs, preceded by (bogus?) out-of-swap messsages).
So, this quick fix raises the question -- can anybody tell us at what point the faulty bootloader will be re-installed during an upgrade?
In the case I am writing about here, I had ended up with an i386 kernel (at least according to uname), so I bailed and re-installed (having full backup of all the config and other modifications). I will be sticking with 11.2 p9 and the "out-of-swap" problem with that box. Fingers crossed that 11.3 will rectify the problem that got me here.
If by bootloader you mean the contents of the MBR and/or freebsd-boot partition, nowhere. You always have to update those by hand. You can easily verify this, /usr/sbin/freebsd-update is a rather large shell script. So anyone can read it.