Tretinoin, or retinoic acid, is a natural derivative of vitamin A. The compound is already produced in your body at some rate and is essential for life. Accutane, no doubt, jacks up the blood concentrations of related compound isotretinoin to levels far above the natural. However, without having researched this recently, I would guess that topical use of tretinoin would not raise your systemic concentrations far beyond the natural levels. I know there is research about this - look for it.
The same cannot be said about Finasteride which 1) is not naturally occurring and 2) has a rather flat dose-response curve where very small amounts have a powerful effect. In the case of Finasteride, I would not advice anyone to use a topical formulation. In the case of Tretinoin, however, I would wager it is fine. I myself use topical tretinion, or rather, used to when I had acne. Now that I have PFS my acne is gone…