Hi Rob,

Sounds like a good idea! In fact, something similar has been proposed by Elliot Olds on the EA Forum: Proposal: Impact List -- like the Forbes List except for impact via donations.

If you're curious about  arguments for (or rather against against) why billionaire philanthropy you might check out this SSC post. This post by  Richard Chappell is pretty good too.
 

I expect much of the data is out there, because the majority of billionaires either want to give publicly, or they need to disclose when they change their shareholdings in their main source of wealth (in the case of the typical company founder) due to regulations, and donating to charity is seen as a good excuse to do this.

It may be rather difficult to gather though, as I don't expect there to be a nice centralised source.

I think this is a good idea as a neutral tracking resource, but I might be against it if it had the effect of heaping additional praise on the billionaires. (I don't like Elliot's Impact List idea.) I think transparency is good.

Perhaps  more effort should be directed at questioning why any human  being needs to be a billionaire?  By encouraging billionaires to donate we are in effect legitimizing their hoarding of vast resources far beyond the needs of any person.  Instead of asking them to give, perhaps we might be discussing how we will reclaim these resources?

Soaring markets helped the richest 1% gain $6.5 trillion in wealth last year, according to the Fed

https://www.cnbc.com/2022/04/01/richest-one-percent-gained-trillions-in-wealth-2021.html#:~:text=%E2%80%9CThe%20pandemic%20wealth%20boom%20certainly,end%20of%202021%2C%20data%20show.