You can record external to hdmi at 1080p. I do this with an atomos ninja blade.
You can even let it trigger the external recorder to record of you enable the in camera h264 proxy recording.
This will give you RAW files in the compact flash card in camera, H264 .mov files on the SD card in camera, And the external recording on the monitor.
There is an app called switch in the magic lantern forum, that can cut out the extra black frames that this workflow gives to the proxies.
This way you can edit with the proxy files in and later sync up the raw files for grading.
I only tested normal 25 fps with this workflow, 48 may behave differently