This is an interesting idea!
The only thing that is not ideal here is that Envato will need to measure LUFS to define the volume. Which is a processing on their side. If they can do that, it worth nothing to simply turn down the level according to the measurements.
So we could modify your idea just a little bit to completely exclude the processing by Envato.
This minimalistic “AJ LUFS Normalizer” app may have a simple built-in FTP feature to it. So you open the app, login into your account, add your preview track, the app measures and normalizes it, then click “Upload”. And this is the only way to upload a preview mp3. Then you should go to the “Upload” page on Audiojungle and do the rest as usual.
This way you can’t skip the normalization process, or do it wrong.
The only issue is that some masters could modify the code of this app to change the target LUFS and be able to upload a louder mp3. Which would be a strange idea but still 
I am not an expert in code so we need one to figure out how to make it impossible (or hard) to modify the app.
Maybe it would be better to make it web-based.