The home we have just brought has no vents in its tiled roof; therefore it is getting a lot of condensation in the loft.
So how do I fit vents?
Do the vent you fix from the inside (rather than putting in venting tiles) work well enough?
A gable louver might be a good option if you don't want to mess with the tiled roof.
I can think of only think of one solution other than going through the tile. you could run a large 6"-8" venting from the crown to the soffits with an in line fan to draw the air down. You would have to make sure that your fan will be able to exchange enough air. I am not exactly sure how to calculate this.
Other than this you would need to put the venting in from the top. Flashing will need to be installed to prevent leaking. I have never dealt with tile, sounds like a big job.