Iron in its oxidized state is not readily usable by plants, so I do not think your strategy would work.

Is there any particular reason you do not want to fertilize the entire aquarium?

 

Well, it's a dirtied tank with MGOPSM and maybe that would be overkill. I'm very unfamiliar with ferts. Might there be a localized option for encouraging the red in this plant, like a tab?

 

you would want to add ferts to the whole tank

dirt is very, VERY good, but stems use up liquid ferts

also, check your lighting. Reds come with higher lights, not with iron.