A different machine has locked up, running recent netbsd-10. I was doing pkgsrc rebuilds in zfs, in a dom0 with 4G of RAM, with 8G total physical. It has a private patch to reduce the amount of memory used for ARC, which has been working well.
All 3 tmux windows show something like [ 373598.5266510] load: 0.00 cmd: bash 21965 [flt_noram5] 0.37u 2.89s 0% 6396k and I can switch among them and ^T, but trying to run top is stuck (in flt_noram5). I'll give it an hour or so, and have a look at the console.