Thanks to sandboxing, the paid version would not be able to access data created by the free version, when trialing.

To avoid this problem in the past, authors offered the free version, and then paid unlock as a separate app package. So you could continue to use the original application you trialed, and the unlock package would turn it into the full version.

Unfortunately, this stopped when in-app purchases appeared.

It feels like an IAP would be more fitting than an entirely separate app, though -- it'd be a shame to require a new listing just to work around this filtering.

Maybe static analysis could determine whether the IAP unlocked a new feature/functionality or not, feeling into the labelling/filtering?

What does this solve?

The developer now has two apps on the play store, still with no paying customers for either of them. The aversion to paying for quality is still present.