So if we can print 9 bits of the printhead, why not see if ESC ^ 9-pin graphics mode works:

[Linked Image from i.imgur.com]

According to the ESC P manual, the second byte's LSB is the 9th pin, but it's actually the MSB, just as in the printer internals.

Code

    1 GOTO 10
    5 X = PEEK( (12*16+1)*256 ) : P = 12*256*16+9*16 : FOR I=1 TO LEN(A$) : POKE P,ASC(MID$(A$,I,1)):NEXT:RETURN
   10 REM APPLE PARALLEL CARD, READ FROM C100 SETS UP STROBE, THEN POKE C090 WILL SEND CHARACTERS


     100 N = 120 * 3 : N1=((N/256)-INT(N/256))*256 :N2=INT(N/256) :? N,N1,N2:A$=CHR$(27)+"^"+CHR$(1)+CHR$(N1)+CHR$(N2) : GOSUB 5
     110 FOR J = 1 TO N/2 : A$=CHR$(1+4+(1+4)*16)+CHR$(128) : GOSUB 5 : A$=CHR$(2+8+(2+8)*16)+CHR$(128):GOSUB 5:NEXT:A$=CHR$(10):GOSUB 5


[Linked Image from i.imgur.com]

Although, come to think of it, this probably worked before the 9-pin fix since it's not going through the shift register 8-)