I never tried it. It would be very useful.

Isn't it quite difficult to draw routes using a tablet? ridewithgps or strava route building works much better with a mouse. I briefly tried ridewithgps on my phone, but I didn't see any way to drag a route to a different road, or select a climb to see it's stats.

Or maybe you'd just be selecting other rider's posted routes?

Transferring a route file:

Download the finished route file to the tablet from strava or ridewithgps.

Google "android copy files to flash drive" for methods. For instance, this page:
How to hook up a USB flash drive to your Android device, but you would need the correct USB connector size on the cable, not a full size connector like a thumb drive uses.

But this page says that "it's complicated". The Android device needs to be the master end, instead of being it's usual slave/receiving end when it's connected to a PC.

It mentions needing an OTG cable, "On-The-Go" to make it work, and hardware support in the tablet.