For seed starting I have been using the fines that I have sifted out of my larger components. When making my bonsai soil part of the process is to remove any material that is too fine in texture. Not wanting to dispose of it I have found that it is fine for starting seeds and cuttings.
This could contain any of the materials that I have used over the years such as, Turface, Haydite, D.E. Lava Rock, Perlite and, for the organic component, Pine Bark or coarse chopped Moss. Still, not a recipe I know but that's what I use.
I do have a concern about your timing. The chilling process you speak of (stratification) is usually begun in late winter in order to allow a spring planting and germination. If your seeds do germinate you are now forced to keep them inside for the winter, not the ideal circumstance.
When beginners ask these types of questions I usually point out how long a process it is to grow bonsai from seed and that you can cut years from the process by obtaining something a little more mature. But since Jack covered that I won't mention it.