Maybe a browser extension with limited permissions? Say the tool looks at Slack and counts how often you use the ROFL emoji. The extension could be granted access to *.slack.com but no other domains.
That could work, but then you have the additional barrier of having to convince people to install your browser extension, and for people who are already worried about privacy, that comes with its own can of worms. Especially if they don't necessarily understand or trust the permission model.