Oh, let me try…

MML → MusicXML → MuseScore → (edit) → MusicXML → [xml2abc.js](http://wim.vree.org/svgParse/xml2abc.html) → prepend four spaces → here:

    X:1
    T:Loreley
    C:Ph. Friedrich Silcher (1789–1860), 1837
    Z:Heinrich Heine (1797–1856), 1824
    Z:MML encoding & arrangement © 2016 mirabilos, published under The MirOS Licence; copyright for text, music, and piano arrangement has expired; music engraving by MuseScore
    %%scale 0.83
    %%pagewidth 21.01cm
    %%leftmargin 1.00cm
    %%rightmargin 1.00cm
    %%score { ( 1 2 ) | ( 3 4 ) }
    L:1/8
    Q:1/4=76.00
    M:6/8
    I:linebreak $
    K:C
    V:1 treble nm="Piano"
    V:2 treble 
    V:3 bass 
    V:4 bass 
    V:1
    "^Andante"!mf! G | ([EG]>[FA])[EG]!>(! ([F-c][FB])[FA] | [EG]3!>)! [DF]2 [DF] | %3
    w: 1.~Ich|weiß nicht, was soll es be-|deu- ten, daß|
    w: 2.~Die|schön- * ste Jung- * frau|si- tzet dort|
    w: 3.~Den|Schif- * fer im klei- nen|Schif- fe- *|
     [CE]3/2 [CE]/ [CE] [B,D][A,C][B,D] | [CE]3- [CE] z G |$ ([EG]>[FA])[EG] ([F-c][FB])[FA] | %6
    w: ich _ so trau- * rig|bin? _ Ein|Mähr- chen aus al- * ten|
    w: o- * ben wun- * der-|bar, _ ihr|gold’- nes Ge- schmei- * de|
    w: greift es mit wil- * dem|Weh, _ er|schaut nicht die Fel- * sen-|
     [EG]3 [DF]2 [DF] | [CE]3/2 [CE]/ [CE] [B,G][B,F][B,D] | C3- C z!mf! E |$ %9
    w: Zei- ten, das|kommt _ mir nicht aus dem|Sinn. _ Die|
    w: bli- tzet, sie|kämmt _ ihr gol- de- nes|Haar. _ Sie|
    w: rif- fe, er|schaut nur hin- auf in die|Höh’. _ Ich|
     ([B,D]>[CE])[B,D] [B,DG][B,D][B,D] | [DGB]3 [CEA]2 [CEA] | [B,DG]2 [DG] [A,^F][B,G][CA] | %12
    w: Luft _ ist kühl und es|dun- kelt und|ru- hig fließt _ der|
    w: kämmt es mit gol- de- nem|Kam- me und|singt ein Lied _ da-|
    w: glau- be die Wel- len ver-|schlin- gen am|En- de Schif- fer und|
     G3- G2 [FG] |$ [EG]>[FA]!<(![EG] ([F-c][FB])[FA] | (([EG]2 [Ge]))!<)! [Fd]2!>(! [Fd] | %15
    w: Rhein; _ der|Gi- pfel des Ber- * ges|fun- * kelt im|
    w: bei; _ das|hat ei- ne wun- * der-|sa- * me, ge-|
    w: Kahn, _ und|das hat mit ih- * rem|Sin- * gen die|
     [Fc]3/2 [Fc]/ [Fc] ([F-B][FA])[FB] | [Ec]3-!>)! [Ec] z |] %17
    w: A- * bend- son- * nen-|schein. _|
    w: wal- ti- ge Me- * lo-|dei. _|
    w: Lo- * re- ley _ ge-|than. _|
    V:2
     x | x6 | x6 | x6 | x6 |$ x6 | x6 | x6 | x6 |$ x6 | x6 | x6 | (B,DE =FED) |$ x6 | x6 | x6 | x5 |] %17
    V:3
     z | (C,/G,/CG,) (C,/A,/DB,) | (C,/G,/CG,) (F,,/F,/A,F,) | (G,,/E,/G,E,) [G,,G,]2 [G,,G,] | %4
     ([C,G,]G,,E,, C,,2) z |$ (C,/G,/CG,) (C,/A,/DB,) | (C,/G,/CG,) (F,,/F,/A,F,) | %7
     G,,G,C, [G,,D,][G,,D,][G,,F,] | ([C,E,]E,,G,, C,,2) z |$ (G,,D,G,) (G,,D,G,) | %10
     (G,,D,G,) (C,,E,A,) | (D,,D,B,) z2 [D,^F,] | D,3- D,2 G, |$ (C,/G,/CG,) (C,/A,/DB,) | %14
     (C,/G,/C/) z/ z (F,,/A,/D/) z/ z | (G,,/G,/C/) z/ z [G,D][G,C][G,D] | ([C,G,]E,G, C,) z |] %17
    V:4
     x | x6 | x6 | x6 | x6 |$ x6 | x6 | x6 | x6 |$ x6 | x6 | x6 | (G,,B,,C, D,C,B,,) |$ x6 | x6 | x6 | %16
     x5 |] %17

Hm, seems to not quite work yet for more complex pieces…