Add shipment reference on invoice

I have a customer who requests to have on the invoice the number of shipments linked as extra references.
It is pretty easy to do by just extending get_origins to append the shipment names of the moves linked to the invoice lines.
So I’m wondering if it should be a standard behavior? For me, the shipment number is redundant with the sale reference because they appears on both document invoice and delivery order. But maybe there are some countries with specific rules about that?

Allready done for our customers. It is very useful if you have a big sale which is shipped and invoiced in different parts. No special rules in germany, just a service for the invoiced company to ease the check between delivered and invoiced quantities.

Same here. Standard behaviour for most installs.

yes, this could certainly be standard.
would this be as additional origin references to the shipment line?

In the meanwhile, we’d also like to put a comment containing a textual representation of the shipment reference as the first line in the generated invoices (based on shipment). But it is not really clear where/how this could easily happen…
[added] and given comments can have origins, how to add the shipment line origin to the list of possible origins for an invoice comment?