First off I want to let everyone know: I did NOT discover this method. My apologies to whoever DID discover it but I don't know or remember who it was.
Someone described this method in one of the many threads about volcano breaching and my thought was, "Whaaaa?! There's no way something like that would work!" So I made a new embark at a volcano and did some testing and, sure enough, this method works.
Here's the basic premise: Miners are able to mine out tiles diagonally above them EVEN IF THERE IS A BRIDGE OVER THEIR HEADS. First you dig out your magma tunnel to feed magma to wherever in your fort you need it and dig it right up against the volcano pipe. Then you channel a trench against the pipe that can be the width of the tunnel if you wish. Build a bridge over the trench making sure to cover it completely and then seal off access to the magma tunnel. Dig a new separate path to access the now bridged over trench. Finally, designate the volcano pipe wall ON THE Z LEVEL OF THE MAGMA TUNNEL for mining. Your dwarves will stand in the trench beneath the bridge but will somehow still mine out the squares diagonally above them, causing the magma to flow safely onto the bridge leaving your dwarves unscathed.
I'm sorry if this is difficult to visualize but I'm terrible at making graphics in discussion threads. If someone who understands this method can illustrate it I'd appreciate it.
The positive points of this method are many;
1. 100% safe. There is no danger to your dwarves whatsoever.
2. Any dwarf can do this, whether Legendary +5 Miner or an unskilled peasant who's never touched a pick before.
3. Can make your breach any size you like. No more having to tolerate slow flow rates from breaching a single tile.
4. If your bridge raises you could use it to stop or redirect the magma flow right from the source if you wished.