This still could be interesting, according to this study, with a less concentrated solution, we could eliminate 50% of DHT in the scalp for only -24% serum DHT as a trade off. For referrence, the usual dose of Propecia 0.25 to 1 mg eliminates 70% of DHT in the scalp but also 70% serum DHT.
If one was tested with high serum DHT before the treatment, I think it's very less likely they would suffer any side effect (at least those caused by the reduction of serum DHT). Pretty sure the level of serum DHT prior to treatment is the key here, if they are low already you're in for a bad ride, which also explains why some get side effects only 24h after popping up the first pill.
The H&W topical has a high concentration of Propecia by default, if you tweak the values down, it might be possible to reduce the chance of systemic side effects big time without losing too much on the effectiveness. I'm gonna probably test that by ordering a solution with level similar to those in the study, but before that I need to get my serum DHT tested.