An interesting hypothesis. As smileyface says it depends on: the ability of the drug, ivermectin, to pass the skin barrier. I am no expert but the skin is a highly resistant body organ, able to resists all manner of things and block them from attacking the human system; IOW, rarely are topical skin meds known to be effective for systemic treatment.
smileyface mentions the skin barrier; I have not encountered that term pharmacologically. It is stated that Ivermectin does not cross the brain barrier. I think the lungs would be another resistant organ to many foreign substances. They are richly profused with blood, so I suppose the question might be how much Ivermectin would be absorbed into the skin and subsequently the blood stream.
One medical substance that the skin readily accepts is steroid(s); I have to watch my blood sugar when taking steroids orally and when I get steroid injections in my spine. Topical steroids have never affected my blood sugar.
Ivermectin would be used orally. The dosage is based on body weight and it is apple flavored. Body weight and flavoring aren’t considerations in topical meds.