First, thanks very much for taking a look at our data! We're very excited about this as well.
We are planning to make improvements to our IATI publication this year. It would be simple enough to add the funder activity ID as a field, but more complicated to gather that data (it's up to our members to provide). If we do add it, more likely than not it will be for new or current projects - not completed ones.
We'd very much like to be part of this traceability discussion. One barrier to traceability I see in IATI - given how few organizations likely use a unique project ID - is the fact that IATI doesn't have a place for you to capture the "original" source of funds. We do capture this in NGO Aid Map, but are not publishing that data to IATI.
So, for example: USAID gives funding to NGO A, who gives funding to NGO B. In NGO Aid Map, we can capture this entire chain (at least in terms of the organizations involved). For the purposes of publishing to IATI, however, we are only "allowed" to report the NGO A to NGO B part of that flow. This is because IATI, for good reasons, wants organizations to only report on the funding they receive or give. The problem with that is that, if - in this scenario - NGO A never reports to IATI, you might never know that the money that NGO B received originally came from USAID. I know this is a bit convoluted, but hopefully it is clear!
By the way - we also have information on who NGO B might be working with (just organization names, not funding amounts), but we have a fair amount of work to do before we can publish that data to IATI.