I am using AG Unit Frames with the "filter buffs/debuffs" option, so that it only shows things I can cast or cure. Is there a way to force it to consider Prayer of Mending as a buff I cast? Currently, it's not showing up; actually, I'm not even sure it showed up when I had all buffs showing either. Any ideas?

I would just use mod called MendWatch. It is perfect thing for the job. Here is the link MendWatch

Thanks, oh fellow Malfurionite, but... MendWatch isn't really what I'm looking for here. Chronometer already gives me a bar timer - I just want a way to see it as a buff icon on someone as well, without having to squint at every single buff they've got.

I would recommend setting up Grid to use a corner indicator set up to display if the target has Prayer of Mending. It is an extremely quick and easy way to get an overview of the current PoM status in a group/raid.

MendWatch creates a Bar on your screen with person's name and number of charges left, moment that person gets hit it displays another bar with same info and colors previous one red and puts (Expired) on it and then fades it out.

[ - Tank 1 - (5) ] - 25 seconds.

after the damage

[ - Tank 1 - expired ] - 0 seconds
[ - Tank 2 - (4) ] - 25 seconds

So no need to squint. It makes very easy way to track who has it. I like seeing the info on fights where multiple people take damage at the same time and bar bounces from character to character. Or you can configure grid to show the buff, but I stick with grid showing relevant debuffs instead.

Oh and hello server, person you.

If you want to highlight specifiable debuffs/buffs on a target, check out Debuff Filter. I use it to show when a mob has Thunderclap, Demo, Sunder, etc so I don't have to search through all the other debuffs.

I use Mendwatch on my Priest, but it seems to get a bit confused when there's multiple Priests casting mending. I still find it pretty handy though.