REQ: auto-generation violates a cross-validation rule but unable to enter the accounting distributio

Here is a problem:   it should be obvious how to do this, but it isn't.

Choose an expense purchase item from the catalog.  The item is set up to default to a specific natural account segment for accounting.

The requisitioner has a default natural cost code segment based on their personal HR / Expense  record.

We have a business cross validation rule where their natural cost code is not allowed against this natural account segment, so we expect to have to modify the accounting distribution to a 'correct' natural cost code.

Having created the requisition line (professional requisitioning screens not iProc), attempt to move to the accounting distribution: are unable to enter the field.  The validation rule violation stops the combination being generated automatically, and so prevents it appearing and therefore stops user being able to edit it to correct it!  This means its not possible to save the requisition and generate a req number.