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I havent done it but can't see why it wouldnt work. The nice thing about grow lamps and growing things indoors is you can control temperatures and light intensity. Should work out better than hot august sun IMO.
I'm not sure if those rootstocks without a single leaf on them would benefit from the grow light. Anyway, I think this is overkill. Putting them in the shade with ziploc bags over the grafts to keep the moisture in would be easier and much cheaper and probably better result.