Hi Robert,
we considered the same approach but this would lead to a problem in the Error Monitor (/Aif/err) whenever you use the transform feature. This does not apply any of the actions so you dont see the complete result structure.
We also considered overwriting the base customizing by selecting a different (higher?) structure mapping number for the variant and reapply all but the conflicting field mappings and fixed values. But this would lead to error prone double maintainance. In the end we developed a function module based value mapping that fetches the specified fixed values directly from the related tables.
Its bothersome to remember that this has to be applied alongside the fixed value and there are certain constellations that arent covered due to the amount of key fields in the mentioned tables but in general this works nicely within our confines.
Kind Regards
David Rittinghaus