I am trying to create a user export preset to create low res jpgs. However whenever I create a preset to do this LR changes is to DNG. I have contacted support and they don't seem too have a clue - they even suggest it's not possible. Any ideas out there?
If you have got "Original" selected as the file type setting for the export, then a straight duplicate of the underlying imported image is exported, not an edited copy. That is necessarily of the same file type as the imported image and has the same starting content (so, under this option alone can a Raw file be exported). It will however be accompanied by a set of Adobe metadata describing your latest LrC edits.
If you instead choose an explicit image file type, then whatever the original image's type might be, a new file of the chosen sort will be created. The bitmap content of this file will directly reflect your latest edits.