I think there is no way to calculate Subset Ratio's using QV objects and dimensions/expressions. Those things typically make use of fields (all values thrown together) while a subset ratio tells you how many values appear in a field instance in a particular internal table.
Well guys thank you all for your help - all helpful answers, alas no possible way to extract that information as is. The only solution I came up with which was useful to a point was a straight table with $Fields as a dimension, and then have a couple of expressions which were count($Fields), count(DISTINCT $Fields) and then count(DISTINCT $Fields)/count($Fields) to show where fields held the same value which is some sort of information integrity measure rather than density. Thanks guys anyway, useful chat.