Certainly it's an old tradition in Riau (and other parts of Indonesia). You pour the coffee from your cup or glass into the saucer and sip it - usually quite noisily - after blowing a little on it (to cool it). This custom is, however, common in polite society.
I always remember drinking coffee with my father at kedai hailam Nirwana in downtown Airmolek during the young days. All people poured the coffee from the cups to the saucers and drank from the saucers!
Teens years a go, I was in a hotel's coffee shop in Bogor with my boss from Riau, who was a Riau’s Malay, drinking his coffee this way eventhough in 4 stars hotel coffee shop! pouring his coffee into his saucer, and then drinking from the saucer. He told me: “awalnya biar dingin saja, lama-lama kalau langsung dari cangkir atau gelas ada yang kurang, tak sedap lah pokoknya!”