A long overdue update, but the experiment using a multivitamin & mineral pill was (not surprisingly) unsuccessful.
I’d currently recommend anyone trying an HOB enrichments starts out by trying to replicate successful ones in the literature like @NarcisP’s Electricity-driven microbial protein production: Effect of current density on biomass growth and nitrogen assimilation. Papers we have found to date use Rabaey, Yu or SL-6 for their micronutrients. They use various mesonutrients similar, but not identical, to the ones I tried above.
See also @arlombardo’s post which alerted me to this oversight.