I never received your answer by mail. I keep getting probe messages from the mailing list as some messages bounce. I have a @yahoo.fr address...
Anyways, thanks for your answer.
I guess it should be possible to write an element based on GstVideoFilter in python and register it statically.
I'll look into that.
What I was looking for is an existing element to which I would connect an action signal that would let me process video frames. But if there is no such thing, I'll go for the GstVideoFilter base class.