For the next guy/gal: to retain (the illusion of) bash as the login shell across reboots, I did the following:
I installed the shellcmd package via the webui.
I added the following "command line" as a "shellcmd": test -x /usr/local/bin/bash && for u in root nu; do chsh -s /usr/local/bin/bash $u; done
I don't touch the default shells /bin/sh and /bin/tcsh. I think the one-liner above is executed by /bin/sh which is very similar to bash (except for differences, which I've never memorized). The one-liner is tested (I did a reboot).
According to the documentation, I could have created something like /usr/local/etc/rc.d/bash-again.sh, made it executable and it would execute on boot. I'm pretty sure I'll go there next, since I want to re-establish other things on boot. In particular, I dislike that ~{root,nu}/.profile seems to be overwritten on startup. I'll be reverting my changes back, ty very much.
If this reads a little hacky to you, well, yes, yes it is. Being new to pfsense, I guess there are some good reasons (control, repeatability) to change the login scripts. I realize this a "router appliance" first and a FreeBSD box second, regardless of my insistence to make it more like the later. But it also surprised the heck out of me. So indulge the noob for talking out of school and bloviating about his hopes and dreams.