Hello Groucho
There are some pretty good studies out there that might help answer your question. Interestingly, most of the studies done with Naltrexone required prior abstinence. Below is a link with a summary of many Nal studies. Here is an excerpt:
Naltrexone does not appear to exert an influence compared with
placebo on maintaining abstinence or in postponing the first drink in
those patients who cannot avoid alcohol. However, there is clear and
consistent evidence that naltrexone is significantly beneficial in helping
those patients who cannot remain abstinent to reduce their drinking
behaviors. They drink less often and in lower quantities, avoiding
full-blown relapse.
http://www.dpt.samhsa.gov/pdf/NTXWPFinalPDF.pdfBest of luck.