Hyslop's words are a very bad fit to the modern StB tune. If somebody was trying to write a new tune for them (why? they're crap) they'd have used a different metrical scheme.

Hanley described how he came to write StB and the occasion of its first performance, but I don't recall him saying anything at all about the tune.

The earliest copy of a tune called Scotland the Brave for the pipes that I can find in Pekaar's index was from:

Piping For Boys, J. Percy Sturrock; Stirling: Eneas Mackay, 43 Murray Place; 1909 (Cannon 340)

which would have been the source for the Boys Brigade manuscript I read in the NLS. Pekaar doesn't do dates very well, though.