#(define-markup-command (addFontFeature layout props feat m) (string? markup?)
   (let ((feats (ly:chain-assoc-get 'font-features props '())))
     (interpret-markup layout props
     (if (member feat feats)
         m
         (markup #:override (cons 'font-features (cons feat feats)) m)))))

#(define-markup-command (removeFontFeature layout props feat m) (string? markup?)
   (define (false-if-empty l) (if (null? l) #f l))
   (let ((feats (ly:chain-assoc-get 'font-features props '())))
     (interpret-markup layout props
     (if (not (member feat feats))
         m
         (markup #:override (cons 'font-features (false-if-empty (delete feat feats))) m)))))

\paper {
  #(define fonts
        (make-pango-font-tree "TeX Gyre Schola"
                            "TeX Gyre Heros"
                            "TeX Gyre Cursor"
                            1))
}

\markup { regular123 \addFontFeature "onum" { add123 \removeFontFeature "onum" add+remove123 add-after-remove123 } }