Hi Mark,
I sometimes use “right hand style” harmonics with my left hand, whenever it is convenient. For example, in an arrangement of a Christmas carol I recently performed in a concert, it was essential to do this, as the harmonics were imitating bells and followed each other in quick succession on the notes C-G-A-E (RH C, LH down to G, RH A, LH down to E over and over!). Playing the harmonics in the same manner with each hand made for clean quick harmonics. This was not indicated any differently in the musical score–I just figured it out. So, I think putting it in a “performance note” would be the best way to indicate this.
Wow, transcribing that Arabesque and Pastorale for 26 strings on a lever harp is a brave undertaking! I wish you the best of luck with it.
Harp Hugs,
Balfour