If possible it will be nice to get this package work with libressl.
Have you tested with libressl?
It's indeed possible, it seems deltup dependency for openssl is just md5 for calculation. I'll take a look over it this weekend.
I tested with libressl and It works. Made a pull request for tree: https://github.com/gentoo/gentoo/pull/4676
It's now in tree. https://github.com/gentoo/gentoo/commit/d4b990da19a0bc779480169b81fe16fb4c8863ff