For me, "zakat being compatible with EA" means "its possible to increase the impact of zakat and allocate it in the most cost-effective way" [ . . . .]

Indeed, effective giving being subject to donor-imposed constraints is the norm, arguably even in EA. Many donors are open only to certain cause areas, or to certain levels of risk tolerance, or to projects with decent optics, etc. Zakat compliance does not seem fundamentally different from donor-imposed constraints that we're used to working within.

I look forward to reading your longer report. If you're interested in convincing skeptics like me, I suggest addressing the following questions:

Can Muslims give Zakat to EA causes? We agree that Zakat and longtermist projects aren't compatible, and that direct cash transfers to the poor are compatible. But what about other cause areas? I'm not familiar with Zakat being used for non-emergency global health interventions (e.g., deworming, reducing lead exposure, deploying insecticide-treated bed nets, etc.). It seems possible, given Zakat use for medical necessities during humanitarian crises (e.g., Gaza), but are there examples that you can provide here outside the context of war or natural disasters? If none currently exist, are there any orthodox Muslim scholars who support this use of Zakat?

What are the specific inefficiencies with Islamic Relief, Helping Hand, and other Muslim-run charities that concern you? You inspired me to do some research, and I've found some worrying reports about Islamic Relief USA "double dipping" on administrative fees, and LaunchGood not actually verifying their "Zakat-verified" campaigns. But generally speaking, do you think the top Muslim charities have higher administrative costs than those of other charities? If so, what's your evidence?

Relatedly: Do you know if GiveDirectly considered their Zakat fund for Yemen a success? Given that 100% of the funds collected were distributed to Yemeni families in need, is this kind of program sustainable?

How will you convince Muslims to redirect their Zakat to non-Muslim charities? I suspect that a significant number of Muslims (including myself) will hesitate to do so. We're fine giving Sadaqah to non-Islamic institutions, but there's a general belief that Zakat should be distributed by, with, and through fellow Muslims. How do you propose tackling that issue?

How will you counter local-first doctrine? We're often instructed that Zakat should be "focused locally to bring about change in the very environment in which the Zakat payers live." (Yaqeen, p. 13) I personally don't follow this rule, but it's a popular one, and it pushes people to give Zakat to their local Masjid instead of internationally, where they could get the biggest bang for their buck. This principle does not appear to apply to Sadaqah, as far as I know, which is a contributing factor for why I think it's easier to pull Muslims into GWWC than to shift their Zakat practices.

Lastly, one response: 

It's not clear yet whether or not the best path to impact would be to first e.g. a) get existing effective orgs to become zakat-compliant, b) get already-zakat-compliant orgs to become more cost effective, c) create a zakat-complaint org which collects donations and distributes them to cost effective orgs, d) create an org which only convinces and promotes effective zakat and then directs people to those orgs without collecting zakat ourselves. 

I don't think this is true. If you can do (B)—convince Zakat-compliant organizations to become more cost effective—so long as their Zakat programming is mostly direct cash transfers, that would be massive impact with minimal effort. You wouldn't need to start a new org and staff it and get it Zakat-certified, or get an existing EA organization Zakat-certified and convince Muslims to trust them.