Bubble and squeek works much better with leftover roast potatoes rather than mash, less sludgy when you've finished. If i wanted to make it into a muffin shape I would cook it first in the pan then stir the browned bits in and then preheat a metal muffin pan with a little bit of fat in each cup, drop in the mixture quicky and then put in a very hot oven for only a few minutes, so the outside of the muffin would get nice and crispy and brown. Then it would hold shape and could be frozen and reheated. the only possible issue I can see is if the mash/potatoes have to much moisture in them they may weep when defrosted and fall apart.
What do you want these things for? Is it breakfast, as a side dish for dinner or a quick lunch?
Hash browns would work if you didn't make them to deep in the tins, only needs potato and onion I use a bit of flour as a binder.
Stuffed cabbage cups. line the moulds with a cabbage leaf, fill it with a flavoured rice mix and fold the leaf over the top, reheat upside down so it holds together.
Same as the above but with a tortilla as the liner would probably reheat better.
Tiny little pies! Use a thin pastry lining fill with any veg/beans of your choice bake and freeze.
Samosa muffin.. line the tin with filo pastry and fill with potatoes/onions/peas. lots of recipes for samosas
online so just make that mix but bake it in a muffin rather than frying it.