Would this make hydrogen fuel cells (the reverse of electrolysis) have double the power density as well? That would be meaningful for reducing hydrogen fuel cell weight and cost per vehicle.
I doubt it. The reason this works is that when combining oxygen atoms from two water molecules to produce an oxygen molecule, they need to have parallel spins; the magnet aligns the spins and therefore speeds up this process. However, when breaking the O2 molecule to combine it with hydrogen (in a fuel cell), the atoms are already parallel in spin and are going to an unordered state, so there's no benefit to attempting to align them.