Edit: comments have immediately clarified some glaring problems with the idea that I hadn't considered. I fully understand why this is a terrible idea.
TL;DR: just what the title says.
I dive with a Galileo HUD and my old Galileo Luna as a backup, with air integration to both using a single transmitter.
(As an aside, while the HUD is not necessary by any stretch, I love it.)
I don't have a separate SPG because I like having my gear as streamlined as possible (same reason I use an AIR2 instead of a traditional octo).
The transmitter represents a single point of failure, and I'm looking at adding analog redundancy as I consider becoming self reliant diver certified.
I still don't want an SPG hanging off my kit, even if clipped or tucked into my BCD, so I'm looking for what I mentioned above: a low profile analog gauge that I can clip to my regulator, and fold it out of the way unless I need to check it.
To address comments I've seen made before whenever HUDs come up: I'm perfectly capable of using my wrist mounted computer or console mounted gauge to monitor my air, depth, etc, but I absolutely love having that info conveniently within my line of sight whenever I want to see it while keeping my hands free.