So, here's the current results of testing;
-The Floodgate continues to burn while in the open position.
-Our frequent rains can extinguish the surface fires and the Floodgate, regardless of its position. So, rain at the wrong time can stop the device; it is recommended it's built close to the entrance to maximize effectiveness because of this.
-Igniting the Floodgate was not simple. I noticed that during my first attempt, the grass lit before the Floodgate (which had magma on the other side). Subsequent testing revealed that simply having magma on one side of the Floodgate didn't manage to get it to start burning.
Taking these into account, here's the 2nd Generation of what I'm going to call F.U.C.K. (Floodgate Unnecessarily Causing Killing) until someone comes up with a better acronym (please do);
Floor One:
, , , -1 (Grass)
║+║ -2 (This is where flammable trash drops)
║X╚═ -3 (Floodgate (To ignite, let 1 unit of magma through, igniting a dropped log, then close the Floodgate again)
║~~~ -4 (Magma Imput (for lighting Floodgate, although this can easily be converted into short-range magma weapon)
╚═══ -5 (It's a wall. Ooooh)
+▲+ -6 (Ramp)
Floor Two:
╔═╗ -1
║=║ -2 (Floor Bars linked to lever; dump something flammable (i.e. all of our trash) here and pull lever to ignite grass fire)
║+║++ -3 (Wow, it's nothing!)
║+║- - -4 (see 3) (- means empty space btw)
║+║++ -5 (it keeps happening)
-▼- -6 (Access ramp)
Another feature (not depicted for obvious reasons) for the 2nd Generation plan is to build a 3x4 set of floors across the brook, muddy it via irrigation, and roof it off so cave moss will grow. This way the fire can spread across the brook, increasing our range.
Of course there's no reason to build this at all, since it requires you to have the basic setup for a magma weapon to do anyway, but damnit, it was fun to design.