The BBC Cymru web page for Catrin Haf Jones’ program has this sentence:
Catrin Haf Jones sy’n codi’r pynciau a gofyn y cwestiynau sy’n bwysig i chi.
So far, in all the 3 Levels of the Northern course, I’ve not come across Noun sy’n Verb as an alternative to Mae Noun yn Verb before (except as the start of a relative clause, or with beth etc).
So, are there rules about when you can use sydd yn as an alternative to mae …?
Yes, if you want to use a focused sentence, for whatever reason. Here the focus is on the presenter, so she is put at the start of the sentence. But that in turn forces you to alter the structure of the sentence, using sydd
(An everyday usage of this structure is announcing yourself on the phone: “Haia, Hendrik sy’ 'ma.” or even “Fi sy’ 'ma” if you can reasonably expect the other person to recognize you by your voice.)