PACO,
Thanks for your kind words. My post that you mention came primarily from my studies with Mahiri Keita (if you're new to this style, he is definitely someone to look up. Not much on him online, but his band has some videos on YouTube and he has produced an instructional DVD through TTM)
As to the bell, my understanding is that the bell plays a supporting roll to the drum. (though there is some disagreement on this) Basically, if the bell part helps you to put the beat in the right place, then it is right. However a lot of teachers give specific bell parts when they are here in the US to help educate the players.
There is another post somewhere in here where we discuss various bell patterns for various tunes. Wassolonka /Ngri comes to mind, also the kenkeni for dununba style has a few "beginner" bell patterns that I have heard.
As to variations on the djembe, I have been taught a variation to one of the djembe parts to Tiriba that Mamady Keita said "the drummers play when they are excited" It is generally not a repeated part, just a little "hick-up" which is thrown in at random-ish. This is the only time I have been taught a specific "variation" for the djembe. It seems like the freedom of the djembe is at least contextual as traditionally, when you learn djembe, you start with only one accompaniment part, and then you get to add things as you go along. After you have proven yourself, you get to start playing solos etc. I would love to hear more about this tradition if anyone has background on it (like how long you would play before getting you first solo etc.
If it wasn't for the fact that it is 1:42 am where I am right not, and my wife is asleep, I would record a quick example of the Tiriba variation.
This is an interesting question for comparative ethnomusicology. I am passingly familiar with the Ewe traditions that you mention. Also Bata has some similar performance practices as far as I can tell (I know even less about Bata than I do Ewe traditions)

Ok... look at a couple more threads then I need to sleep!
C