Is there something that I can drag to my hotbar that will fire my rune arm? I'm not finding anything.
Yes, there is.
You can make a hot bar fire you runearm by remapping one of the keys in the UI.
On the main menu, go to "options"
Select "key mapping"
search for "rune"
Hit the big "use rune arm" button to the left and the next key you press will remap to using the rune arm.
I've done it a number of times. I usually map it to a key (like '1') in my rotation.
The only thing (at least in my experience) is that it remaps that key for all your characters. It's easy to map it back when not using a runearm.
There may be other ways to do this, but that's the only key I've ever remapped.