[Ecwid Made Easy.com]

I thought it was possible to order out of stock products if you:

Step 1
Make sure "Hide out of stock products" in [system settings - cart] is not ticked

Step 2
In stock control for each product you need to control the "Quantity In Stock".

For those products that can be back ordered set this to "unlimited".

For those products that cannot be re-ordered select "Limited To" and enter your stock quantity.

All that is missing from Ecwid is a label that says "On backorder" when the quantity reaches zero, similar to "out of stock". Perhaps some custom javascript would do the trick in the meantime.