Rather than go into a lot of specifics ... let me just give you some approaches to take. Specific fills will best come from a book(s), listening to authentic recordings of the style, and experimenting with what sounds good to you. I know you know all of this ... but I'm saying it anyway for the
Newbie New to Ñañigo ... say THAT three times fast!!!
The beautiful thing about
Afro-Cuban 6/8 and 12/8 grooves is that they are made for POLYRHYTHMS. Finding the common denominator in these meters quickly shows you that any grouping of 2, 3, 4 or 6 will work VERY well since they are all multiples of 12.
Authentic fills used in the Ñañigo style incorporates this idea; poly-phrasing as well as polyrhythm. Also, the use of the synopated
bell pattern (show below with the
bold print numbers) will flow smoothly with either one of these multiples.
Bell Pattern: 1 2 3 4 5 6 7 8 9 10 11 12
Play the bell pattern with one hand while tapping out 2, 3, 4 or 6 with the other hand (or one of your feet). Incase the
multiples are confusing, just think of everything in eighth-notes. To tap out 2 you would play every other eighth-note, tapping out 3 you would play every third eighth-note ... and so on. Doing so will line up perfectly with the bell pattern ... perfect for fills, solos and
metric modulations. Playing duple and triple figures, broken and sequential, are also common traits found in
authentic stylings.
For an unconventional approach ... do whatever you want as long as it sounds good!

The fills that I use in my
Afro-Cuban 6/8 Groove - Part 1 on-line lesson is a good example (I think anyway) of the possibilities of getting outside of the groove but remaining in the style. My approach is definitely more
back=beat oriented, but it's an Afro-Cuban 6/8.
Start singing fills ... being sure to notate them. Do some math equations and see what you can squeeze in the span of time allowed with the groove.
Now this may seem a bit over simplified, but to describe specific fills would took volumes ... which I know you all realize.