I found this on eBay and I figured this is just what My Opus 88 Demonstrator needed. It's branded FANMU, though none of the few sellers on eBay who offer it market it as such.
I always found eyedroppers to be inadequate for the purpose of filling their namesake pens; not enough capacity, a hassle to clean... boring...
The Fanmu addresses all these shortcomings, IMHO. It basically works like the filling mechanism in the TWSBI Go and it works really well. It's fun to use and it's quite well made. Looks good too.
It's one of those things that you didn't know you needed, but once you have one you wonder how you managed without.