After doing a search for it on another forum (search here turned up nothing) it seems that Topical Tacrolimus fights the immune reaction, and although it sounds like that would be key, it seems that its results for hairloss were not very impressive.
Cyclosporin is a drug that does very much the same thing but is far more effective. Aparently this drug grows hair very well, though can be hard to absorb topically as its a large molecule. But the real reason this drug is not commonly used for hair loss is that it is VERY DANGEROUS! Systemic absorbtion can disable your immune system, which can lead to you getting very sick, and possibly even dying.