You could maybe try cineform but that is not free, other than that I do not know. Worst case you can do DNxHD using the following work flow, you can keep 90% of the resolution.
1.) In AE set your comp size to 1440 x 1080 with pixel aspect of HDV 1080/DVCPRO HD 720 (1.33).
2.) Set the pixel aspect of your footage to 1.33 then scale your footage to 90% :(.
3.) Export out to DNxHD at 1920 x 1080 using media encoder.
4.) Import in premiere and set the pixel aspect ratio to DVCPRO HD 1080 (1.5) and set the sequence settings to 1920 x 1080 with a 1.5 pixel aspect ratio.
This will give you a final resolution of 2880 x 1080, its not 3200 x 1200 but it is better than 1920 x 720 and should be very efficient to work with and edit since it is just HD footage with a non square pixel aspect. For step number 1-2 you could just set the comp size to 1920 x 1080 and scale x by 120% and y by 90% instead, I prefer to use pixel aspect ratios when working with anamorphics since that is essentially what an anamorphic lens is changing. It keeps the number of real pixels lower and in turn requires less memory. The 1.33 aspect in AE combined with the 1.5 in premiere will give you a 2x stretch in the end. Just make sure that AE and Premiere are set up to display the pixel aspect ratio correction and everything will look as expected.
If you go the DNxHD route you can use the extra resolution to slightly reframe your shots before exporting to DNxHD and not scale as much or at all, or you can shoot at a lower resolution in ML like 1472 and save some memory.
On somewhat of a side note I always try and keep my footage the same resolution until the final output and use pixel aspect ratios to deal with the anamorphic stretch. So if you can find a codec that will export 1600 x 1200 keep the footage that size and just assign a 2.0 pixel aspect to it when working with it rather than scaling it to 3200 x 1200, it will reduce the number of pixels you are working with by half compared to scaling. I always keep my masters this way and then export out square pixel / black bar versions as needed when I go to the final format like HD.
If you find a good alternative codec please post, I am sure many would like to know as well. Good luck.
Maybe keep an eye on this for the future, it appears it will be open source based on cineform.
https://kws.smpte.org/kws/public/projects/project/details?project_id=15