I'm currently transcribing some baroque guitar music. For some pieces the performer is expected to improvise a rhythmic ornamentation, e.g., for a notated quarter note you are free to play several shorter notes (that together take the same time as the quarter note). Although I know that baroque music grants a lot of freedom to the performer, the rhythmic ornamentation for these sections is especially important. Playing the pieces in the way they are notated would rather sound dull and boring and is certainly not the way the composers intended their music to be played.
This leads me to my question: Is there some more or less commonly used notation to convey this kind of expectation for rhythmic ornaments in sheet music? I would rather use some symbol or indication that has some kind of tradition instead of inventing something.
I already know of the indication "ad lib". However, I believe that this indication refers to an even greater freedom of interpretation, and in fact I already use it in the sense of the second meaning listed in the corresponding Wikipedia article ("to improvise a melodic line fitting the general structure prescribed by the passage's written notes or chords").