T3 and T4 addition to a topical delivery system would be very effective, especially since these compounds are quite amphiphillic in and of themselves, making local delivery as opposed to systemic delivery all the more likely. In fact the whole issues of using an amphiphillic carrier such as benzyl was brought to Nimni's attention after a 1970's review showing that amphiphillic molecules, including thyroid hormones, have significant local accumulation.
Whether it is necessarily that effective I do not know, but it is in any case quite complementary and possibly synergistic with most products currently used for topical fat loss, and given the low price of T3, I can't see any harm in adding it to such a formula at all. You are going to get a very effective yield of T3 in the subq tissue locally and it will exert some effect in fat loss, complementary or possibly synergistically to lipolytic fat loss compounds.
I would go with T3 however. T4 to T3 conversion usually drops during a diet, making direct T3 administration a more effective manner.