Just noticed that DeGruy's 17-string guitarp is, indeed, a modified 7-string guitar. It has seven tuning machines and seven strings arrayed across the fretboard in addition to the ten "harp" strings.

Thanks, GUEST,Jim, for taking a stab at that answer. Sounds plausible, and I'm willing to believe that your memory is probably pretty close to correct, if not absolutely right. If Spider John's modification was indeed a "do-it-yourself" job, drilling a hole dead-center in the headstock for a banjo peg would have been a reasonable approach.