There is a LibreSSL overlay in Gentoo, which patches various software that would otherwise fail to build with LibreSSL, currently they are these packages:

   dev-qt/qtcore
   dev-qt/qtnetwork
   net-misc/socat

It used to be a bit painful to setup, but things have improved, now I only need this in make.conf:

  USE="... libressl"
  CURL_SSL="libressl"

And this to mask it: /etc/portage/package.mask/openssl

  dev-libs/openssl

There is still some software that would fail to install (with a conflict between openssl and libressl), but none that I need currently.