Is it possible for a substance to be absorbed by the hair or the scalp?
As to the subject line, yes, lipid substances or any non-polar small molecule will enter the skin cells and the body below. The reason is because cell membranes simply consist of fatty substance that behaves like a solvent for other lipids.

As to the specific question about this product, Monovin A. This is a veterinary product and the producer, Laboratórios Bravet, intends it to be injected into animals in case of a vitamin A deficiency. The main ingredients is a retinol ester in the amount of 2,000,000 IU (which usually means retinol equivalents in this respect).[1]

Indeed, retinol is a lipid and long-chain esters like retinol esters are non-polar, too, and can readily be de-esterified by esterases, making vitamin A (=retinol, retinal, retinyl palmitate, retinoic acid) immediately available to the surrounding cells and, via transport in the blood stream, to the liver, which is the body's store for it. Daily doses of 25,000 IU are known to be toxic when administered over a long time. Doses of 50,000 IU can be dangerous over weeks. In general, it appears that single high doses are better tolerated than long over-exposure.[2][3] So, do the math and dilute the stuff such that you get at most one hundredth of an ampoule, else...

Now the effects. Indeed, hair loss is one of the symptoms of heavy vitamin A-deficiency, and logically, that symptom can be cured with vitamin A. That does not mean that more results in more. While retinoids are important therapeutic agents for the management of many skin diseases, notably acne vulgaris, psoriasis, ichthyosis, and palmoplantar keratoderma, there exists something like retinoid-induced effluvium which is a side effect of such medication with tretinoin (ATRA).


  Retinoid-induced hair loss (telogen effluvium) is among the most
  frequent and psychologically most distressing adverse effects of
  retinoid therapy, which results in premature termination of a
  clinically desired and often highly effective systemic therapy with
  retinoids.[4]


Since ATRA is readily produced from retinal, its concentration would be expected to be up, even if the original stuff was retinyl ester. In summary, I would expect no effect, or even hair loss, for you don't simply put such potent stuff on your head and elsewhere, and have no reaction from the body.

[1] http://www.bravet.com.br/pr-ceg-monovina.asp

[2] Vitamin A: Toxicity from Supplements In: Medical Sciences Bulletin 1995; Pharmaceutical Information Associates, Ltd.

[3] Hartmann S, Brørs O, Bock J, et al.: Exposure to retinyl esters, retinol, and retinoic acids in non-pregnant women following increasing single and repeated oral doses of vitamin A. In: Ann. Nutr. Metab.. 49, Nr. 3, 2005, p. 155–64. doi:10.1159/000086879. PMID 16006784

[4] K. Foitzik, T. Spexard u.&nbsp;a.: Towards dissecting the pathogenesis of retinoid-induced hair loss: all-trans retinoic acid induces premature hair follicle regression (catagen) by upregulation of transforming growth factor-beta2 in the dermal papilla. In: The Journal of investigative dermatology. Vol. 124, Nr 6, June 2005, p.&nbsp;1119–1126. DOI:10.1111/j.0022-202X.2005.23686.x. PMID 15955085.