OK. Here we go. I found some time last night. Attached is a git patch file for changes that implement this feature request as well as (GNU) diffs for the individual files in question. It's a pretty simple commit. It's worth noting that I'm probably overlooking something in how I create the glyph, I definitely don't understand the nuances of the display code. But I hope it's helpful.

On Mon, Sep 13, 2021 at 12:27 PM Eli Zaretskii <eliz@gnu.org> wrote:

> From: Michael Gallagher - NOAA Affiliate <michael.r.gallagher@noaa.gov>
> Date: Mon, 13 Sep 2021 12:17:09 -0600
> Cc: Lars Ingebrigtsen <larsi@gnus.org>, 50506@debbugs.gnu.org
>
> Funny enough, that's *exactly* what I tried to do, but I ran into problems getting the variable to display in
> customize-variable (a problem with what I put in cus-start.el?) and if I manually used setq it didn't seem to
> get translated correctly by the way I coded the macro.

I think if you look up all the pieces that handle
display-fill-column-indicator-character, both in C and in Lisp, you
will have your answers. Feel free to ask specific questions if
something is still unclear after that.