You are correct in that you would not be able to use the FC7 with the method that I suggested using the external amp and speakers. There are a variety of stereo volume pedals that have the left and right inputs and outputs like I described. I believe I am using one by Boss, but I'll have to check it. I do know that there are two kinds, related to the overall resistance or impedance of the pedal. It's been a while since I bought mine, but I believe one type is for regular line-in levels, like what we would be using with the output of our keyboards, and the other type is for use with an electric guitar or other device that outputs a significantly lower level signal. If you use the wrong type of pedal, then it will either be too sensitive, where just a small amount of pedal travel will go from silence to blasting, or not sensitive enough, where even full pedal travel would have little effect.
I have not noticed any real noise issues with the pedal I am using now, but I did notice noise with a previous model. It was just a lower quality pedal that started going bad. Also note, however, that most of the music I play is rock, pop, movie/TV themes, and other louder types of music, so I don't usually have very quiet passages where such noise would be more noticeable.
I took a look at the link you provided for those MIDI pedals. Honestly, I know very little about MIDI. I was wondering if having some sort of interface with MIDI could control the volume, but I don't think that overall keyboard volume is a MIDI function. It seems to me that the volume of the notes being played by MIDI is dependent on the data being sent with each note, and the volume settings of the device that is playing the MIDI data. I didn't see anything about overall volume control in the advertisement in that link. However, maybe someone with more knowledge of MIDI can weigh in on this.
Finally, I imagine that you've been playing the keyboard and have determined that a pedal would be beneficial to you. However, have you tried just experimenting with the touch sensitivity? I've played a couple live gigs recently, and I didn't use my pedal for the gigs. I was using a set-up where I stand up, so the only pedal I used was a sustain pedal on my PSR-E433 (I also use a Roland Gaia SH-01 synth with it). And I didn't really miss the pedal -- I just use touch sensitivity to control the volume of the music while playing. However, as I said before, I mainly play music that is typically a moderate to loud volume, without too many dynamic changes throughout the songs, so that would also play a part.
I'll post again once I get a look at the pedal that I'm using -- as well as the one that went bad -- and let you know what brand/model those are if that information is printed on the pedals.
*** EDIT ***
I checked -- I am currently using a BOSS pedal, but I could not find a definitive model number on it. The old pedal that started getting noise is branded as a company called Daphon. Again, no clear model number on it. I remember the jacks were pretty close together, causing the plugs to fit tightly and push against each other -- maybe that eventually caused a bad internal connection, leading to the noise.
I did a quick check on Sweetwater's site, and there are not too many stereo volume pedals there, but there are a few. And they distinguish between "volume pedal" and "expression pedal". I'm not sure, but I think a volume pedal is what I've been describing -- something to attenuate the audio signal before it goes to the amp. And I think an expression pedal is meant to hook directly into a compatible keyboard or other musical instrument, where it controls selected parameters on the instrument directly. At least, that's what seems to be the case.