Mmm... not exactly what I was looking, but nice approached and probably as useful as I wanted. So, you set a constraint on the 'lot number' forcing it to be unique (if the option is checked for that product). Is the code available at Launchpad or elsewhere so I can try it?

Yes, but that would imply to create copies of the same product model for each reference/serial number. That would result in a huge list of products, with same characteristics but the reference. But the idea is to have only one product defined with its characteristics, and then be able to assign a 'lot number' (one or more units) and, if needed, a 'serial number' (unique number for each unit). As this is usual in manufacturing, so you can track a single unit or its whole lot/pack.

It can be done by customizing the current workflow and some extra code.