What's the rate of companies doing this?

I have no real idea, but I wouldn't be surprised if it is less than 1%.

I finally emailed my bosses asking our company to give about 100$ a month spread over various projects. I thought not to ask too much as 100$ infinitely much more than 0.

We're not a very big company, could easily afford 1k a month though, but then it becomes an actual sum that's harder to motivate as we're mainly working with MS products.

The problem I ran into is that my company will match employee contributions to registered non-profits. Wireguard doesn't seem to be a project under a registered non-profit. My company won't donate to a random Patreon or PayPal link.

There have been numerous threads here on HN about this: donating to people to support open source development does not match up with what companies usually do, so it runs into all sorts of corporate friction. Whereas a monthly or annual support contract is the sort of thing that companies do all the time, and at low dollar amounts it might not require much approval at all.

Unfortunately it is up to open source developers themselves to set their business model, and many don't seem to like (or want to do the work of) setting up overtly commercial relationships like support contracts.

I would place a bet on not greater than 0.1%