Yes, you can. It's called contributing to a project. If the "half million lines of diffs" were actually things needing fixing, then the upstream team would accept them. If they are not necessary changes (such as ripping out all windows compatibility), then no, they would reject such changes.
It will take years, maybe a decade before a new ssl library becomes the "default". OpenSSL has a lot of ground covered and a lot of history. Yes, it's common knowledge that libressl started before heartbleed, but the reasons for the project being started are mostly along the lines of:
1) We don't think upstream would take these changes
2) We don't like some aspects of the design philosophy
3) We can do it better.
All 3 reasons can be collapsed into a more focused effort to fix the already existing and very good ssl library; openssl.