This article by Stijn Bruers called "Clean technology innovation as the most cost-effective climate action" states this:

"Funding research and development of clean technologies, in particular clean protein and clean energy, is probably the most cost-effective method to avoid dangerous climate change." 

I think Stijn is arguing that funding clean protein is similar in cost-effectiveness to top Founders Pledge charities that work on clean technology. The article doesn't talk about Giving Green's recommended charities, but GG's recommended charity Clean Air Task Force is also recommended by Founders Pledge. 

My guess is that donating to clean meat charities like New Harvest or The Good Food Institute could be as good as donating to Clean Air Task Force or other recommended Founders Pledge charities. And it's likely better than the other GG-recommended charities like The Sunrise Movement, Tradewater, Climeworks, and BURN, since some of these recommendations have gotten criticism here.

Last August Stijn wrote a post titled The extreme cost-effectiveness of cell-based meat R&D about this subject.
Let me quote the bottom line (emphasis mine):

This means one euro extra funding spares 100 vertebrate land animals. Including captured and aquaculture fish (also fish used for fish meal for farm animals), the number becomes an order 10 higher: 1000 vertebrate animals saved per euro.
...
Used as carbon offsetting, cell-based meat R&D has a price around 0,1 euro per ton CO2e averted.

In addition, as I wrote in a comment, I also did a back of the envelope guesstimate model to estimate the cost-effectiveness of donations to GFI, and arrived at $1.4 per ton CO2e (and $0.05-$5.42 for 90% CI).

It is important to mention that our methods are not nearly as thorough as the work done by Giving Green or Founders Pledge about climate change, and I wouldn't take it too seriously. Nevertheless, I think that it at least hints the order of magnitude of the true numbers.

Edit: I just realized that Brian's comment refers to a newer post by Stijn, which I assume reflects his broader opinions. However I think that the discussion in the comments on Stijn's older post that I linked to is also interesting to read.