JackAss is a VST plugin that provides JACK-MIDI support for VST hosts.
Simply load the plugin in your favourite host to get a JACK-MIDI port.
Each new plugin instance creates a new MIDI port.

Here's JackAss loaded in FL Studio:


And an example setup in Carla for it:

JackAss sends the notes from the host to its JACK-MIDI port.
It also exposes 50 parameters, which send a MIDI CC message when changed.
You can use this to easily control external applications that accept JACK-MIDI input and possibly CC for automation (like Carla).

Additionally there's a JackAssFX plugin, which only exposes parameters to send as MIDI CC, in case you don't need MIDI/notes.

JackAss currently has builds for Linux, MacOS and Windows, all 32bit and 64bit. Just follow this link.
As a bonus, you also get special Wine builds - load it in a Windows application running in Linux via Wine and you get a real, native JACK-MIDI port from it!

PS: Why JackAss? Because it outputs to JACK. ;)