Update! Here’s a workflow that should solve this for you:
The key is to load/sync the workout in the EBC app while you still have a network connection, before starting your ride.
Here are the steps that worked for me:
- Turn on your Karoo with Wi-Fi/network enabled
- Open the Xert EBC app from the Extensions menu
- Go to Activities → Workouts and confirm your workout is listed at the top
- Exit the EBC app
- Start your ride from your Karoo profile (Wi-Fi will turn off automatically)
- Tap the MPA or XSS data field to relaunch EBC
- Open the menu (☰) → Workout — your workout should still be there and ready to start
That’s how it worked for me when I tested it 
Note: You won’t hear interval notification sounds from EBC on Karoo — we don’t currently have access to the device speakers.