I looked up Steward in Merriam-Webster and found the following definitions:
1 : one employed in a large household or estate to manage domestic concerns (as the supervision of servants, collection of rents, and keeping of accounts)
2 : SHOP STEWARD
3 : a fiscal agent
4 a : an employee on a ship, airplane, bus, or train who manages the provisioning of food and attends passengers b : one appointed to supervise the provision and distribution of food and drink in an institution
5 : one who actively directs affairs : MANAGER
I don't really get the idea of a person playing a piece of music being its "steward," but I kind of like the idea of performing musicians and composers being like note stewards. Composing musicians manage the harmonic and melodic concerns of the music while it is being written (would you like another E-flat in that chord, can I make that sextuplet more comfortable to play by changing the articulation, would this make more sense if there were five beats in this measure?), and performing musicians direct the affairs of the notes and phrases (I think that the G-sharp is the most important note in this measure, or that low D could be softer, or this is the right tempo).
Which reminds me of a totally unrelated story (or maybe it is related): Samuel Coleridge-Taylor was having a composition lesson with Charles Villiers Stanford, and Stanford spilled some of his tea on Coleridge-Taylor's score. He was then reported to have said "now your piece is in the key of ti."