this is what I learnt: the only thing its getting from that first preset is the GUID for the editing mode.
If the GUID relates to custom (in the adobe editing modes.xml file), then the preview will be set to i-frame 1080.
If I hack the red 4k preset to this:
<PreviewPresetVideoCodec.Win>1096180840</PreviewPresetVideoCodec.Win>
<PreviewPresetFileName.Win>DNxHR HQ UHD 25</PreviewPresetFileName.Win>
<PreviewPresetVideoCodec.Mac>1096180840</PreviewPresetVideoCodec.Mac>
<PreviewPresetFileName.Mac>DNxHR HQ UHD 25</PreviewPresetFileName.Mac>
<EditingModeGUID.Win>CA2629AB-945F-43C5-9FB2-C4A304C54F9E</EditingModeGUID.Win>
<EditingModeGUID.Mac>CA2629AB-945F-43C5-9FB2-C4A304C54F9E</EditingModeGUID.Mac>
then the new sequence will be DNXHR. I cant find an editing mode though that uses GOPRO cineform.
Anyone know of one?
I did it! 
So neil you were right that the alphabet comes into it:
premiere looks for the first preset that corresponds to the sequence. It looks up the editing mode only from that preset, then it uses the first sequence preview preset it finds in that editing mode's folder - in alphabetical order. so the hack (and this is for me more than anyone else) is:
1. hack 4K HD 16x9 25.sqpreset with this:
<VideoAllowLinearCompositing>true</VideoAllowLinearCompositing>
<VideoUseMaxRenderQuality>false</VideoUseMaxRenderQuality>
<VideoUseMaxBitDepth>true</VideoUseMaxBitDepth>
<VideoPixelAspectRatio>1,1</VideoPixelAspectRatio>
<PreviewPresetVideoCodec.Win>1128679492</PreviewPresetVideoCodec.Win>
<PreviewPresetFileName.Win>Quicktime GoPro CineForm (YUV 10-bit)</PreviewPresetFileName.Win>
<PreviewPresetVideoCodec.Mac>1128679492</PreviewPresetVideoCodec.Mac>
<PreviewPresetFileName.Mac>Quicktime GoPro CineForm (YUV 10-bit)</PreviewPresetFileName.Mac>
<EditingModeGUID.Win>9678af98-a7b7-4bdb-b477-7ac9c8df4a4e</EditingModeGUID.Win>
<EditingModeGUID.Mac>9678af98-a7b7-4bdb-b477-7ac9c8df4a4e</EditingModeGUID.Mac>
Premiere only looks at the GUID, but without the other stuff the file>new>sequence will spit the dummy.
2. Find the folder called "9678af98-a7b7-4bdb-b477-7ac9c8df4a4e" (ie the Custom editing mode folder). On my system it's here:
\Program Files\Adobe\Adobe Premiere Pro CC 2017\Settings\EncoderPresets\SequencePreview
3.Duplicate Quicktime GoPro CineForm (YUV 10-bit).epr and rename it 1a-Quicktime GoPro CineForm (YUV 10-bit).epr
This might seem like a massive waste of time, but I love my workflow. I have keyboard shortcuts setup, and having each clip as a new sequence means I can add warp stabiliser at their native resolution and pan and scan on a 1080 timeline.