Are you using pegged shop orders for the items on the customer order?  Or invent order supply?  How are you currently matching the supply and demand - is it through MRP or Order Point or through manual intervention?

Depending on those answers, there could be a different answer, but I don’t know of an out of the box solution that would allow an alternate to be selected based on a sales part.  It would have to be done through and event or an update to check the Planned/Released Shop Orders and update their operations list according to how they are linked to the demand.  But again, if that is done through pegged shop orders, you would arrive at one solution, whereas supplying from invent orders would be a bit different.

We are using the “shop order” sourcing option on the sales part. So the customer order line launch automatically the shop order. There is no manual intervention, order point or MRP use.

An event seems to be difficult in such a case...

OK, that is good as there is a direct tie then back to the sales part on the order.  Assuming you also have no automated shop order processing in place, the Shop Order stops at Planned, so it is possible at that point to have a scheduled event once a day or whatever frequency is necessary to find Shop Orders in Planned with Inventory Part 1 as the made item and then update the routing according to the connected sales part from that line on that customer order.  Because the Routing update is a standard api run from the shop order header RMB, the sequencing of all the steps can be mimicked to perform the update.

You may be right that it ends up being too many actions for the event, but I’m sure it is doable as a custom method if it can’t be done as an event.  It may require a Custom table though to maintain the mapping of Sales Part to Alternate, that depends on whether you have just one part like this or multiples.