I would probably use it frozen and macerate as in the intstructions.
When the rhubarb thaws it will become soft and release its juices, but isn't that the purpose of macerating? If it looks liquidy, you could hold the milk and add to make the batter as liquid as you would like for muffins.
I've used frozen rhubarb in cakes without altering the recipe, as well as pies and chutneys.
The muffins sound delicious. Let us know how they turn out.