My employers have generally been fine giving me blanket permission to contribute to specific open source projects.
The framing matters: don't say "can I please do some charity work because it makes me feel good".
Say, "can I have your permission to get free rigorous review from experts in my field, and zero out all future maintenance costs for your company by contributing my fixes to the upstream open source project?"
Because that's really how it is. No employer of mine has ever said no to that. It is entirely in their interest for you to do this, you just have to help them see it.
The framing matters: don't say "can I please do some charity work because it makes me feel good".
Say, "can I have your permission to get free rigorous review from experts in my field, and zero out all future maintenance costs for your company by contributing my fixes to the upstream open source project?"
Because that's really how it is. No employer of mine has ever said no to that. It is entirely in their interest for you to do this, you just have to help them see it.