Bowed strings
Bowed string instruments (notably violin/viola/cello/bass) can easily and almost unlimitedly play microtonal music, and in fact are easier to retrofit due to the lack of frets. Unfortunately, most trained players of the instruments are going to be finnicky about playing in any way other than the way they're used to, thanks to the pedagogy. There are two important tools in making microtonal string music work, scordatura and fingerboard marking.
Techniques
Scordatura
If you're going to be using the traditional notation system or a variant of it, specify that the strings be tuned to the scale system in question's versions of A, D G, etc. to give the players familiar notational home bases to work from. Try to make the scordatura somewhat distant from the correct pitches for maximum effect, however, because broken-in string instruments "ring" for the notes they play often (in 12), and the scordatura will have the effect of disorienting the string player's instinctual seeking of the resonance.
Fingerboard marking
Tapes are the most awesome (because they're color codable), but chalk works too. Players respond to them well because most of them learned to play with tapes in elementary school. Provide tape to the players, debunking any myths that it will affect the sound or damage the instrument. If it does leave some sticky residue, tell them to wipe it off and suck it up. If you don't trust your string players' ears, or the part is technically challenging, color coded tapes are plainly the easiest way to get it in tune fast.
Bowed fretted strings
The viol family (viola da gamba, some types of violone, baryton, etc.) of the Renaissance and Baroque eras consisted of various sizes of bowed fretted instruments. The frets were tied on lengths of gut, which could be moved to adjust for temperament, with the provision of split frets being possible to enable playing distinct sharps and flats. The arpeggione is a Nineteenth Century instrument made according to a similar idea, and experimental fretted bowed instruments have been made in modern times (GuitarViol, fretted violin, etc.), although most often not with movable frets.
Music
Most (if not all) of Ben Johnston's string quartets are written for bowed strings in just intonation.
Many more examples could be added here.