Thanks to the fiscal sponsorship arrangement I mentioned with the Freedom of the Press Foundation, you already can: https://freedom.press/crowdfunding/signal/

That money will, minus whatever portion the Freedom of the Press Foundation keeps to cover its overhead, be spent on Signal. And donations for Signal to this foundation are tax-deductible to the extent 501(c)(3) charitable donations usually are.

I have no connection with any of the people, projects, or organizations we've been discussing. But I am the president of another fiscal sponsor 501(c)(3) nonprofit in the free/open source software space, so I'm very familiar with this model.