Is it too late for a cover crop in our greenhouse?

This weekend we just finished pulling out the spent plants from this years greenhouse crop of peppers, tomatoes, eggplant, cucumbers and basil.  We have gotten in the habit each year, normally around mid December, to pull all the spent plants and trellising out of our greenhouse so that we can plant a cover crop.  (We use cattle panels for trellising.)   For the past several years we tilled the earth, seeded our cover crop of cereal rye and crimson clover, then we watered the cover crop in.  During the winter, without any heat, we get pretty good growth from our cover crop which adds decent organic matter and a source of nitrogen back to the soil. 
Last year we did an experiment of seeding our cover crop directly into our rye mulch without any tilling.  We were amazed at how great the germination was that this year we did not till the earth but seeded our cover crop directly onto the rye mulch. 
It will be an interesting experiment to see if we get enough growth before early April when we transplant these crops for another season of wonderful veggies.  Every year we consider growing a crop of greens for our “winter garden” but decide that we need to re-furbish the earth.  Someday we will grow for ourselves in our greenhouse for a continuous source of greens throughout the winter!