The master-to-slave part is pretty easy. Whatever program is on the host computer just needs to send a serial commands to your arduino (on windows it's usually referred as a COM port). The arduino just has to wait for serial data, then read the command and do what the command says. Same way you would interact with reading input from the serial monitor.

As for the hardware, My first thought was a small spray bottle that you can put various smelling liquids into, then use a servo to push down on the little sprayer.

Pure alcohol is often way superior to a re-wash, since it dries very quickly and the emulsion returns to its full hardness. If you're in the middle of a print (or scan I guess) and find some hair or dust that won't blow off, you can nudge it free with a fine artist brush dampened in alcohol. If you use water on the neg, you're out of business for a couple hours.

If you get a little cosmetic spray bottle, good alcohol (I have iso, methanol and everclear - grain alcohol) gets crazy-useful - lenses, computer screens, glass neg carriers or scanner glass, framed pics, etc.

If you print fiber or make darkroom masks, methanol is the shizz. It's part of dry-iodine bleaching which can let you bleach out very tiny spots on prints, or lighten up things like strands of hair or details in plants or textures - pretty immense control with some practice. You can use it to fine tune masks or even bleach out or lighten parts of negatives - and it will redevelop if you screw up (though on a print, the tone may be very warm). It's been a game changer for me (but I do a lot of lith printing where sometimes you just get a black spot for no reason). You can retouch prints to remove actual details and then use spotting dyes to correct the blank spot.

It's my understanding that ISO HEET - the fuel-line de-icer - is 99% iso, and that the regular HEET is 99% methanol. I use the regular stuff for iodine "dry" bleaching and it's just fine - F me, methanol was hard to find until someone on Photrio pointed me to the auto parts store. (The MSDS docs for these products state 99% for each).

You can get 99% at Frye's (in the US) in the PC building dept.

If your state sells 190 proof grain alcohol, that's 95%. I use it for liquid emulsion projects from time to time, and it's another good one for cleaning film, optics, etc.

Pro tip - get a little cosmetic spray bottle for whatever alcohol you use. Freakishly handy! Great for glass like computer screens or (big-time!!!) glass negative carriers or scanner glass.