This one comes along every year some poor soul finds and tries the recipe.
I have heard of people who claim it works and makes a liquor type drink but it is no more 'rum' than elderflower champagne is the real thing. Most who have made the attempt say that it is not just a waste of several pounds of demerara sugar and time, it also makes a sticky mess which takes even more time to clean up.
The marrow has a high percentage of water so if you have a juice extractor, put the marrow through that and mix the juice with brown sugar and try to ferment it with a yeast.
Pure alcohol does not freeze. To test how strong a home brew is, put a measured amount in the freezer over night then in the morning measure the unfrozen liquid on top and work out the percentage and it will tell you how strong the brew is. It spoils the taste and will be totally unfit to drink, so only freeze a small amount.