I was going to suggest using a memorized Transaction. Just like a Scheduled Reminder, it can do what you ask for.Just memorize a correctly categorized transaction, then locate it in Tools / Memorized Payee List. Edit the split category amounts down to 0, save the entry and enable the "Lock this entry" flag, to prevent it from ever being changed by a future transaction.
but it would be simpler if you could just have a way to move the line items up and down the list.
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