So previously I dropped some theories on alcohol, well here are the facts. I have focused on Isopropyl alcohol which was the main constituent in Yvonne's tonic. This alcohol is a 'proper' alcohol for lack of a better term (unlike the fatty alcohols like cetyl alcohol and stearyl alcohol).
The good news
Isopropyl alcohol is thought to evaporate off skin with 15-30 seconds. It therefore is not thought to absorb into the hair. A study on skin similarly agrees that is is not absorbed (Unwanted effects of cosmetics and drugs used
in dermatology By Anton C. de Groot, Johan
Pieter Nater, J. Willem Weyland)
I am however not certain what cleaning effect you would receive with the product evaporating so quickly.
The not so good news
Soaking hair in alcohol for several hours can weaken it. Note that several hours means 24 hours which is a longggggg time. This weakening applies to alcohol concentrations of around 50 -70% in water and is thought to be related to a mix of changes to the keratin fibre and dehydration of the hair (Nature 184
, p444, 1959 and J Soc Cosmet Chem, pp 695-704, 1977). Typically hair tonics are within this range.
Above 50-70%, hair soaked in alcohol does actually become stronger. (This does not mean purer alcohol is better, remember that studies are done on cut pieces of hair in the lab!)
As a final note, if you are using alcohol tonics, do be careful as inhalation can be mildly irritating to dangerous depending on the individual (Journal of Emergency Medicine pp 687-692, 1997).