I play the classical guitar(mainly because I was raised playing guitars) and the flute, and I find it strange that Pius x would mention the guitar in general terms specifically(as not allowed), yet allow a small bowed instrument(the fiddle). Yes, the modern guitar(even during pius x time was a profane modern instrument). But, in the guitar family of instruments, there are some legitimate candidates(small baroque guitar). And, you can construct it in such a way to be in my opinion acceptable. The modern fishing line nylon strings with technologically advanced metal wrapped strings and metal frets are certainly modern and a "no go". But, an older more traditional guitar or instrument like it(lute) can have gut strings(like the acceptable bowed instrument) and frets made of gut. This was the traditional setup. It even says in the mass "Confitebor tibi in cithara" - "I will praise thee on the harp". In essence saying, "I will praise thee on the plucked instrument". And, the guitar is definitely a descendant of the harp. And, that is in the very mass itself.
It is good that pius x brought sacred music to our attention, but I think that this one got past him. The bowed instrument is not free of criticism either with sayings like - "the devil rides on a fiddle stick". Yet, pius x approved of it. There is plenty of talk of what is appropriate at mass with many fiercely insisting that the pipe organ be used. But almost all, if not all of the pipe organs being used for masses are computerized and electrified modern contraptions. Fr. Cekada is currently even begging for funds for a new organ because theirs is not technologically advanced enough(that is his punch line - 30 year old electric outdated technology is unbecoming). I guess what I am saying is, people are missing the point.
In sum, it would be very easy to construct a traditional 2 octave(the same as the human voice) guitar type instrument to rival the accepted fiddle to accompany holy mass.