Foolish bastards!
My foolproof method (well not really mine I read it elsewhere):
Magma channel level.
XXXXXXXXXXM
CCCCCCCCBXM
XXXXXXXXXXM
X=Walls
M=Magma
C=Channel
B=Bridge
On the top level, I have the channel and at the square right before where I intend to breach, I build a bridge after channeling down. I prefer to use non-magma-safe material for this. Then I wall off the channel level so nobody can get in.
The breach level.
XXXXXXXXXXXXXX
UCCCCCCCCCCCRX
XXXXXXXXXXXXXX
U, Upward staircase
R, Ramp
This is in the z-layer directly below the magma channel. If you don't like a leftover ramp, I think you can deconstruct the ramp originally present and construct a new one out of non-magma-safe material. It doesn't matter what direction the hallway comes in so long as the ramp is directly below the bridge.
Now double check and make sure the magma channel is inaccessible. And designate the X directly to the right of the bridge on the channel level. Watch as the dwarf goes down to the corridor under the magma channel, stands on the ramp, and digs upward diagonally into where you want to breach.
When he's out, build a floor on the stairwell to keep anyone from going in there.
The channel will fill up, the bridge will melt, and you're done.
If you're really concerned with safety you can build out of magma-safe material but I don't like having the ramp and bridge around. The dwarf will have plenty of time to get out, though, unless that "idiot dwarf decides to fall asleep" thing happens, so make sure you have enough beds. Always make sure of that.