No, there's no selective upload. Anything you add to LRCC on mobile will automatically get uploaded. Having said that, you could temporarily pause syncing until you had finished culling and deleted the rejects. Then resume sync and only the keepers should be uploaded. Bear in mind though that you may have no backup of those images until they are uploaded (e.g. any picture taken on the phone using the LRCC camera will only exist within the LRCC app on the phone until it is uploaded), so if you do pause sync, remember to turn it back on asap.
You could disable 'Auto Add Photos' in the Mobile App General Settings, shoot with the mobile device's native camera. In Camera roll, delete the rejects. In Lr Mobile make an album, click the menu bar ( ... ) and choose Add Photos, choose 'Camera Roll'.
From there you will have another chance to 'Select' which photos you want added to the Lr CC ecosystem.
A few extra steps, I know. But you actually get 2 chances:
1 to delete rejects,
2 to 'Select' photos.
You would have to give up shooting .DNG but using the Native Camera you have a better camera IMO - Video, for example.
If you want to 'develop' images from within the mobile device, Adobe supports an app called Aviary. It's listed right on their page of available apps.
Thanks for the replies. I'm using a trial of LRCC mobile and the pause feature does exactly what I want to do.