Well, i guess that's similar to a programmable DC-DC converter. It can be called adjustable, and the Vref pin will be used to set the output voltage. I didn't wrap my mind around it first, though i had seen a "reference pin" on some of them. So i think they differ in just that programmable have sort of higher level interface, like I2C, instead of Vref.
To use a RC circuit to smooth the control signal, i already know how. (The formula F=1/(2piRC)).
Maybe i'll eventually try it, but if i understand correctly, an adjustable converter will still require a lot of parts around it, which aren't easily in the datasheet. Maybe also an operational amplifier to bring the Arduino's 5 V to the needed 12 V.
....
Also difficultating the matter is that i'd need a buck converter as close as possible in output voltage to the input voltage. They seem to return at best 0,9. (Mosfet in this aspect is 0,97. And i can't yet find that high.) Or buck+boost, but they are only resistor-selected.