I've embedded wolfssl sources into libbitcoin-network. This is quite a large library, but has an integrated openssl compat layer, and a GPLv3 license, which allows it to drop in to the boost::beast::ssl::stream template. A large number of the source file can be removed once it's all working and we know exactly what the requirements are. Given the openssl interface, it should be possible to replace it with another implementation or even roll our own behind the interface at some point.