I've been experimenting and playing around with notchless hearthboards with the bowdrill (there may have been a time when notches weren't used due to limitation in ancient tools...) I tried just drilling into wood but couldn't get an ember as no concentration of the dust except around the edge - I think it is possible. if you go for long enough (and I have seen photos of embers around the edge of the hole). But there are other methods which work well... I tried the below techniques :
1) I drilled one hole with the bow drill, then I drilled another next to it so the dust fell into the adjacent hole and formed an ember (this worked on the first attempt.)
2) Using a thinner spindle (about 5mm) I drilled a hole on one side quite deep until nearly through then directly opposite drilled another hole with a wider spindle, and an ember formed within the hole. I pushed the ember through with a stick (this also worked on first attempt.)
There are other methods, one being: placing two sticks horizontal side by side and tying them together at each end, and then drilling in the middle of the groove and an ember should form at least on one side, sometimes both sides. I've not tried this one yet. I also tried drilling close to the edge of the board, but the drill kept slipping off.
The above two methods, did work well however.