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

#(define-markup-command (with-font-feature layout props feats m) ((lambda (x) (or (list? x) (string? x))) markup?)
    #:properties ((font-features '()))
    (if (string? feats)
        (interpret-markup layout
                          `(((font-features . (,feats . ,font-features))) . ,props)
                          m)
        (interpret-markup layout
                          `(((font-features . ,(append feats font-features))) . ,props)
                          m)))

\markup \with-font-feature "smcp" "abcd"

#(define-markup-command (with-font-feature layout props feats m) ((lambda (x) (or (list? x) (string-or-symbol? x))) markup?)
    #:properties ((font-features '()))
    (define (false-if-empty l) (if (null? l) #f l))
    (define (assert-string s) (if (string? s) s (if (symbol? s) (symbol->string s) (format #f "~s" s))))
    (if (string-or-symbol? feats)
        (let ((feat (assert-string feats)))
           (if (equal? (string-take feat 1) "-")
               (interpret-markup layout
                                 `(((font-features . ,(false-if-empty (delete (string-drop feat 1) font-features)))) . ,props)
                                 m)
               (interpret-markup layout
                                 `(((font-features . (,feat . ,font-features))) . ,props)
                                 m)))
        (let* ((string-feats (map assert-string feats))
               (remove-features (filter-map (lambda (x) (and (equal? (string-take x 1) "-") (string-drop x 1))) string-feats))
               (add-features (filter (lambda (x) (not (equal? (string-take x 1) "-"))) string-feats)))
          (interpret-markup layout
                            `(((font-features . ,(false-if-empty (append add-features (lset-difference equal? font-features remove-features)))))
                              . ,props)
                            m))))

\markup \with-font-feature "smcp" \with-font-feature "-smcp" "abcd"

\markup \with-font-feature #'(smcp lnum) \with-font-feature #'(-smcp lnum) "abcd"