Tile vents

I read somewhere that tile vents could be made out of ordinary cement tiles. Anyone know how?
My roof is in marley major tiles and I'd prefer not to have to use a plastic tile vent with a box sitting up looking fairly ugly. The ones with the drain boxes below will be a pain to fit, cutting battens sarking and avoiding the rafters. Does anyone have a neat solution to this? I cant use a ridge ventilator either unfortunately.

Thanks.

Hi Check the link it may suit your needs. Good Luck
http://uk.youtube.com/watch?v=6Cetmi6iCY4

That's helpful, sure but it'd be nice if anyone's got a clue if I can turn a concrete tile into a vent directly, maybe with a Stihl saw? But how to stop rain getting in too?