Turns out & ~ does work.

Decided not to expose this as a datastore option for the Client,
but it can be used internally to toggle the compression.

lib/rex/socket/ssl_tcp.rb

@@ -89,7 +89,9 @@ begin
       self.sslctx.ciphers = params.ssl_cipher
     end
 
-    if not params.ssl_compression
+    if params.ssl_compression
+      self.sslctx.options &= ~OpenSSL::SSL::OP_NO_COMPRESSION
+    else
       self.sslctx.options |= OpenSSL::SSL::OP_NO_COMPRESSION
     end
 

lib/rex/socket/ssl_tcp_server.rb

@@ -139,7 +139,9 @@ module Rex::Socket::SslTcpServer
     ctx.options = 0
 
     # enable/disable the SSL/TLS-level compression
-    if not params.ssl_compression
+    if params.ssl_compression
+      ctx.options &= ~OpenSSL::SSL::OP_NO_COMPRESSION
+    else
       ctx.options |= OpenSSL::SSL::OP_NO_COMPRESSION
     end