I thought your methods were crazy, but rather than dismiss is out of hand, I ran some numbers.
Turns out its a lot less crazy than I originally thought.
I compare the mineral content of a multivitamin (Centrum Women) to the content of Seachem Flourish. I excluded nitrogen, phosphate, chloride, and iodine because it seems unnecessary for our purposes. The doses I compared was 1 pill of Centrum Women vs 5 mL of Flourish
Potassium
Flourish: 28.5 mg
Centrum: Not included
Calcium
Flourish: 7 mg
Centrum: 400 mg
Comparison: 57x greater in Centrum
Magnesium
Flourish: 5.5 mg
Centrum: 64 mg
Comparison: ~12x greater in Centrum
Boron.
Flourish: 5.5g
Centrum: Not Included
Cobalt
Flourish: 20 ug
Centrum: Not included
Copper
Flourish: 5 ug
Centrum: 1 mg
Comparison: 200x in Centrum
Iron
Flourish: 16 mg
Centrum: 7 mg
Comparison: 2.3x more in Flourish
Manganese
Flourish: 2.4 mg
Centrum: 5 mg
Comparison: ~2x more in Flourish
Molybdenum
Flourish: 45 ug
Centrum: not included
Zinc
Flourish: 35 ug
Centrum: 8 mg
Comparison: ~230x
Some things to point out. First, Iron is the highest demand ion for plants, so that's where you're most likely to see deficiencies. To dose Centrum 'iron equivalent' to Flourish, you need to dose Centrum at 1 about pill per 20g. Second, at an iron equivalent dose, you will have 500x the copper dose and 600x the zinc dose. This cold be disastrous for shrimp or other more sensitive inverts, as they are sensitive to copper. I'm not sure about zinc or the other ions I listed. Third, centrum is missing several micronutrients (Mo, Co, B) that are needed in very small amounts. This might not be an issue, but is worth noting. Fourth, is is notably lacking potassium, which is a fairly high demand nutrient for plants.
A lot of my apprehension comes from the other things included in Centrum, eg, all the vitamins and binding agents. Their interaction in the tank is a big unknown factor in a tank as far as bacteria, algae, etc, are concerned.
So really, there are some notable drawbacks and uncertainties that come with this, but its not an entirely unreasonable thing to do. It's better than nothing, but I would recommend that you get good root tabs (or use osmocote) that are balanced such that the nutrient contents are more in line with what plants need rather than humans (huge difference metabolically) .
This is all scratch math, so there might be errors.