With the introduction of premium select on transcon flights, it looks like Delta has updated it's rules for upgrades with regional upgrade certificates:

• Flights with domestic Delta One and domestic Delta Premium Select:
  • If you purchase Main Cabin or Delta Comfort+ and then apply your Global Upgrade Certificate, you will be upgraded to Delta Premium Select upon request and then will be added to the waitlist for Delta One and eligible to clear starting 24 hours prior to departure. If you apply your Regional Upgrade Certificate, then you will be upgraded to Delta Premium Select pending upgrade availability.
  • If you purchase Delta Premium Select and then apply your Upgrade Certificate, you will be upgraded to Delta One, pending upgrade availability.

From the reading of this, it looks like RUC's are no longer eligible to be used for upgrades to Delta One on transcontinental flights