Do corporate OSS projects welcome major non-employee contributions?

Do company-backed OSS projects accept significant contributions from non-employees?

As a seasoned developer with no prior experience in Java/JVM projects, I've noticed it's often a requirement for open roles at companies and projects appealing to me. So, I'm looking to learn the language and ecosystem while building something I can point to if/when job hunting.

To give a specific example, extending Elasticsearch's query language with new features (or fixing bugs to get started) seems like something I'd enjoy working on.

I know the company drives development, and the project's open-source status is disputed -- and, honestly, I'm not too bothered by that.

Here's the question, though. Do they actually accept larger contributions from non-employees? Say I pick a feature from their query language roadmap and offer to implement it -- any realistic chance it actually gets assigned to me? (The repository has "help wanted" issues, but they mostly seem minor and already claimed).

What's keeping me from asking directly in a relevant GitHub issue is the need to familiarize myself with the codebase before I can offer to help with something specific. Given its size, that could take a while. So the idea is to find a popular project welcoming serious contributions from outsiders before committing time to study its code base.

Not fixated on Elasticsearch specifically, that's just an example of an exciting project. Maybe they're saturated with contributors, and I should look elsewhere?

Appreciate your experiences and anecdotes around this topic.

Thank you.

PS: I'm under no illusion that every employer cares about an applicant's open-source work. That's fine as long as the OSS project is interesting to hack on.