It's certainly technically possible, though it needs some thought about how causes and actions get contributed and vetted.

It should probably have a geographical component so people can coordinate on local issues.

It could also use a way for people to at least sign in and say they did the action they committed to. Asking for them to prove they did the action is probably too much to ask, though in the case of writing letters, they could post the letter. They could post pictures of themselves showing up at demonstrations.

Getting people to know about it and use it is a coordination problem in itself. What it' would be like if it's non-partisan would be interesting, and it would also be interesting if it's set up to be partisan.

I suspect people don't have a clear idea of how many people they want to make a commitment before they'll do something, but maybe thinking about the question is useful in itself.

If it catches on, it will need to be financed.