Creating new invoice line w/o copy control

Let me run a scenario by you and you tell me if you can think of a quick way to achieve this.

We have substitutions on a Sales order that come in automatically. Customer order 1000 cases of A, we don't have it,so we sub in 1000 cases of B.

The original line item gets TAPA item cat, the substituted item gets TAN.

RIght now we didn't design the TAPA item cat to copy to the delivery or the invoice. Changing that impacts downstream interfaces so it was proposed to handle this during the invoice creation process. It was decided to hack up the invoice creation process using a user exit and go back to the SO and grab the original line item and copy it down to the invoice. This is necessary because the pricing for the substituted item is based on the original item. We give a discount if the new item is more expensive then what they ordered, etc. If we dont have the original item, it throws this off. It's also necessary for invoice print.

Just want your thoughts on how this could be accomplished technically. My fear is doc flow is ruined, we may miss updating a table somewhere and who knows what other dependencies could be out there. Curious if anyone else has encountered a similar situation and can recommend approaches.