Good question. I don't have an answer out of my head, but I'll look into it. This is, however, likely a piece that we are missing 

Untested by me, but it seems the workaround is to inherit from EntryRewardApplicator, and then inside the protected methods you can add information to the AffectedEntries, which each contains a list of PriceEntry and each PriceEntry has an ILineItem. Not sure if it works, but it looks like it will 

Thanks Quan.  I have something half working, needs more work and testing but as an initial implementation this is what I have.

    [ServiceConfiguration(Lifecycle = ServiceInstanceScope.Singleton, ServiceType = typeof(EntryRewardApplicator))]
    public class CustomEntryRewardApplicator : EntryRewardApplicator
    {
        private static readonly Injected<IContentLoader> _contentLoader;

        protected override Decimal ApplyPercentage(AffectedEntries item, Decimal percentage, PromotionProcessorContext processorContext)
        {
            var returnVal = base.ApplyPercentage(item, percentage, processorContext);

            // Update custom properties on lineitem
            PriceEntry priceEntry = item.PriceEntries.FirstOrDefault();
            if (priceEntry != null)
            {
                // Retrieve the campaign
                if (_contentLoader.Service.TryGet(priceEntry.Promotion.ParentLink, out SalesCampaign campaign))
                {
                    priceEntry.ParentItem.Properties["PromotionCode"] = campaign.Code;
                }
            }

            return returnVal;
        }
    }

NIce. However the default implementation is already registered via ServiceConfiguration, so if you want to make sure your implementation gets used,you have to register it in IConfigurationModule instead.

Just FYI: I am working on something that will make the override easier. But let's see if that gets enough approvals from my colleagues 

OK thanks Quan.  This seems to be working correctly for me, so for the time being will stick with this, but if an easier override does make it into a release I'd be keen to upgrade.