I tried it last night with a batch of fresh raw milk kefir but the whey from kefir didn't look quite the same as the whey from raw milk, sitting at room temperature for 4 days, and obviously it didn't taste as strong as the raw milk whey. So I abandoned it. I just have to wait for the next 4 days for the raw milk to be ready then I'll just use that to make whey.
The whey from kefir and from raw milk taste differently. I prefer the whey from raw milk more, it's stronger and has a sharper color. I was hoping to save time, but I guess I have to either make whey from yogurt or from raw milk sitting for 4 days.