Sorry about that. If you like, you can cook up your own proxy presets for formats that are not currently available. Are you aware of how to do that? It's not commonly known how.
You do that in Media Encoder by creating a new Ingest Preset. For an Ingest Preset, you first need an Encoding Preset to create it.
Make a new Encoding Preset in the Preset Browser based on what you want for your proxy. Then create an Ingest Preset from the Encoding Preset you just made. Back in Premiere Pro, in the Create Proxies dialog box, you can browse to this new Ingest Preset, select it, and choose it, and you'll be set up for your proxy with its custom aspect ratio for this and all future projects.
Please return with any questions. I hope the info helps.