I've discovered this year that not only can you add layers inside your shoe, you can add them on the outside. Wearing shoe rain covers over my winter shoes helps my feet stay warm longer. They block the wind and create an additional pocket of air for insulation.
I bought a larger pair of rain covers and plan to either line them with wool from an old sweater, or use the arms from the sweater to make a huge pair of socks that go over the shoes.
My craft talents aren't great so I'll do whatever is easier (or can coerce mom into doing). Can you use a hot glue gun to attach wool to nylon?
Another thing I rarely see mentioned is foot position on the pedals. I used to have my cleats fair far forward so there was more pressure towards the front of my foot when pedaling. Moving the cleats back reduced the pressure and improved circulation to the front of my foot. The same should also apply when using platform pedals.