The smallest package I'm aware of commercially is 3.5mm SMT. It could be mounted on a break-out card to wire it up, but that's pretty complex. Another option to consider is wiring up a 5mm WS2812B addressable LED and hooking it up to a flexible light pipe (example)
with a 3mm lens. Although flexible, light pipes don't do real sharp turns too well.
I had a similar problem with my build. I wanted my SLI and Mono blocks to be RGB, but the LED ports on the blocks were 5mm and 3mm (respectively). I was dealing with analog RGB, so I was also concerned with total current load on my headers. My solution was to cut RGB LED strips to the appropriate size, and attach to the edge of the blocks with silicone sealant.
Worked well for me, and was cheap. (your wife may appreciate that).