Right, here we go...
the panic happens when setting up the loopback interface as I initially suspected. I've added the addr2line info next to the backtrace addresses below.
The key seems to be in ifioctl_common() at src/sys/net/if.c:2988
Starting network.
[ 32.8299972] Fatal kernel mode data abort: 'Alignment Fault 1'
[ 32.8299972] trapframe: 0xcc7d1bc8
[ 32.8299972] FSR=00000001, FAR=cc7d1ddc, spsr=60000013
[ 32.8299972] r0 =c29b23c0, r1 =cc7d1dcc, r2 =00000000, r3 =00000000
[ 32.8299972] r4 =c04f1e5c, r5 =00022078, r6 =bfffdfd0, r7 =00000010
[ 32.8299972] r8 =bbc0d180, r9 =bfffe0a8, r10=bbc17b65, r11=cc7d1c44
[ 32.8299972] r12=cc7d1c48, ssp=cc7d1c18, slr=c04f9798, pc =c04f1a2c
Stopped in pid 95.1 (ifconfig) at c04f1a2c: strd r2, [r1, #0x10]
db> bt
0xcc7d1c44: c04f14e4 ifioctl_common /home/michael/netbsd/src/sys/net/if.c:2888
0xcc7d1c74: c04f9798 loioctl /home/michael/netbsd/src/sys/net/if_loop.c:523
0xcc7d1d74: c04f2350 doifioctl /home/michael/netbsd/src/sys/net/if.c:3248
0xcc7d1da4: c045d68c soo_ioctl /home/michael/netbsd/src/sys/kern/sys_socket.c:212
0xcc7d1e84: c044dd48 sys_ioctl /home/michael/netbsd/src/sys/kern/sys_generic.c:671
0xcc7d1eac: c001aa28 sy_call /home/michael/netbsd/src/sys/sys/syscallvar.h:65
0xcc7d1edc: c001ab10 sy_invoke /home/michael/netbsd/src/sys/sys/syscallvar.h:94
0xcc7d1f8c: c001b058 syscall /home/michael/netbsd/src/sys/arch/arm/arm/syscall.c:250
0xcc7d1fac: c001ad34 swi_handler /home/michael/netbsd/src/sys/arch/arm/arm/syscall.c:172
addr2line -e sys/arch/evbarm/compile/obj/NSA320s/netbsd.gdb -f c04f1a2c
ifioctl_common
/home/michael/netbsd/src/sys/net/if.c:2988
I noticed that the addresses vary slightly for each kernel build (I should probably enable MKREPRO and all that), so will re-run the same tests tomorrow and check whether the bt is any different.