"An excellent artificial Wine like Claret"

This isn't usually a cookery (or brewing) blog but I'll make an exception for a recipe from Hannah Woolley's The Gentlewomans Companion or, A Guide to the Female Sex: The Complete Text of 1675, Prospect, 2001, p. 198.

An excellent artificial Wine like Claret, but much better, and by many degrees brisker

Take two gallons of your best Sider, (some esteem Worcestershire Red-streak the best) and mingle it with six gallons of water, put thereunto eight pound of the next Malaga Raisins bruised in a Morter; let them stand close covered in a warm place, for the space of a fortnight, stirring them every two days well together; then press out the Raisins, and put the liquor into the same vessel again; to which add a quart of the juice of Raspberries, and a pint of the juice of black Cherries; cover this liquor with bread, spread thick with Mustard, the Mustard-side being downward, and so let it work by the fireside three or four days; then turn it up, and let it stand a week, and then bottle it up, and it will taste as quick as the briskest liquor whatever, and is a very pleasant drink, and much wholesomer than French-Wine.

The "brisker" bit I'd certainly believe, but I am puzzled by the mustard - I can't imagine the yeast would like that? I'm almost tempted to try it - I wonder if anyone has come across a report of someone doing so?