Don't the stairs and travelator from the W&C platforms emerge in the
ticket hall on the street side of the barriers? (I could be wrong, I
hardly ever use them).
Given that there are now no barriers in the foot-tunnel between the W&C
platforms and the LUL part of the station (and even when there were,
they were rarely in use) it would be possible to get to the DLR that
way.

The W & C has a gate line at the foot of the travelator, and if you're
using prepay you should use a validator at Waterloo, so that isn't the
answer....