\version "2.19.80"

% this is a broadcast-function for single events like \p or c''4
#(define (broadcast-music context music)
   ; get attributes for this music expression
   (let ((evcls (ly:assoc-get (ly:music-property music 'name) music-descriptions '())))
     ; log time and music
     (ly:message "~A ~A ~A"
       (ly:context-property context 'currentBarNumber)
       (ly:context-property context 'measurePosition)
       (with-output-to-string (lambda() (displayLilyMusic music))))
     ; broadcast event
     (ly:broadcast (ly:context-event-source context) ; broadcast to this context
       (ly:make-stream-event
        (ly:assoc-get 'types evcls '()) ; get event class from attributes
        (ly:music-mutable-properties music)) ; get properties for this music-event
       )
     ))

% 3 tests
evA = \p
evB = \once \override NoteHead.color = #red
evC = g'4

% a demo engraver
eng = #(make-engraver
        ((start-translation-timestep trans)
         (let ((context (ly:translator-context trans)))
           (cond
            ((equal? (ly:context-property context 'measurePosition) (ly:make-moment 1/4))
               (broadcast-music context evA)) ; on moment 1/4 broadcast \p
            ((equal? (ly:context-property context 'measurePosition) (ly:make-moment 2/4))
               (broadcast-music context evB)) ; on moment 2/4 broadcast \override ...
            ((equal? (ly:context-property context 'measurePosition) (ly:make-moment 3/4))
               (broadcast-music context evC)) ; on moment 3/4 broadcast g'4
           )))
        )

\layout {
  ragged-right = ##f
  \context {
    \Voice
    \consists #eng
  }
}

% demo music
% Why are the overrides not applied?
\relative {
  \repeat unfold 3 { c''4 c^"p" c^"red?" c^"+g" }
}