Looking online it seems the red lava rock does contain more iron than black lava rock but how accessible it would be to that plants doing what you're thinking of doing is something I'm not knowledge on. When dosing iron it seems people use chelated iron which from what I understand is specifically to dose in a form that makes the iron accessible to the plants. Powdered chelated iron is not that expensive and dosing it at say 1/8 t. every day or so for something like a 60-08 gal aquarium it would last a long time. (I got mine from this guy: www.aquariumfertizer.com
That said how are you dosing your micro nutrients now? If using something like TMG, Flourish, or CSM+B you are already dosing iron, and if so, what makes you think you need to add more? There's some speculation I've read that too much iron can cause certain stringly, thread-like algaes....some say this is not true, others insist it is.....I started dosing some extra iron 2-3 weeks ago and in the last few days for the first time see thread algae. Am going to stop with the iron and see if it changes anything.