How to forecast components?

Hi All

We have a business requirement to be able to forecast components (and component sub-assembles) that will be eventually be used in an end assembly but not sold on a sales order.

The detailed requirement is as follows:

1. We make highly configured to order machines - more like design to order / one off type machines.  As such there is no way to have a forecast for these items as we have no BOM or idea on what will be required/made in the future.

2. The machines are generally 80% the same but vary in key components, but we want the ability to forecast the 80% common parts as we know that these will be required