falkTX from KXStudio has just added LV2 support to Non Mixer. This is not upstream at the moment but is available in the KXStudio repositories. This will hopefully be accepted upstream and become part of Non Mixer. In the meantime, you can try out the LV2 supported version if you are using KXStudio by simply updating your system.
There is no support for custom GUI's and falkTX points out that there may never be. Non the less, this allows you to load up and use LV2 plugins in Non Mixer.