As far as the magma fall goes, I think you'll still have at least 1/7 Magma on the ground once the fall stops which will hurt the dwarves.
I like the idea though. I have a similar plan in my head based off the magma/water falls in "The Incredibles" where the fall is "split" to allow entrance.
I saw one fort that had some permanent magma falls but he would let some bridges down to umbrella a particular couple of tiles and then stack his goblin cages under the bridge and flip the lever again to incinerate them.
One thing to bear in mind - what happens when two dwarves use the system at once?
* Dwarf A steps on a plate, disables the trap, and walks on to the bridge.
* Dwarf B steps on a plate, re-enables the trap while Dwarf A is still on the bridge, and then walks on himself.
* i_population = i_population - 2;
Plates don't normally work like that. It sends an "on" message when something meeting the conditions steps on it and an "off" when they step off.
Floodgates and bridges have a delay so it's very easy to get them mixed up by sending signals too quickly- thus this whole idea needs some big revision to work very well.
However I can say that creatures will pretty much try to path through water or magmafalls- this will be extremely deadly to invaders walking over some steel (or whatever) bars but just as deadly to any dwarves. The liquid is only for a very brief time present on the tile they want to walk through and even if it is with a two or three tile wide waterfall they'll think one of the other tiles is open when they check.
A more complicated toggle is required do deal with these behaviors, and probably with some more instant toggle mechanisms like mechanisms, doors, or floor hatches.
There are a number of ways in which you can have the pressure plates link of to some water flow related device elsewhere and then have it result in a more on-off triggering of another plate so as to open and close the waterfall for a longer duration.
However as with most waterfalls the perils of overflow should not be underestimated. Sending a constant stream of magma about will be quite hazardous if it backs up thanks to magma's lack of pressure without a pump driving it. I would recommend a two level drop below the bars and to have always active pumps situated to suck it right out of those low tiles and into the other vent or into a reservoir for the magmafalls.
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If you like complex contraptions you could make sure that the magmafall would only trigger for ambushes and sieges by having a small field of pressure plates each hooked up to a different mechanism that connected a power source to a pump for another plate and that it took at least four inputs at any one time to drive the pump thanks to other junk connected to the same gear-axle complex.
Things can only take damage from magma if magma is sitting on the same tile as them. So a closed door will hold magma back but as soon as the door opens if it's not magma safe it might as well be made of tinder.
There are a few weird exceptions to this.
Wooden pumps will slowly or rapidly take damage from pumping magma- it varies.
Also glass buildings seem to have a higher melting temperature than glass. While a glass block, giant corkscrew, and tube will all melt when exposed to magma a glass pump doesn't deconstruct with magma sitting on it. If I recall the test correctly a built glass statue will survive magma while the item version will not.
I guess I should try and find it so I can find out if glass grates stand magma...