As an example, if I:
- create a Bezier curve
- add a Shrinkwrap modifier to the Bezier curve and target some other object with it
- add a Bezier Circle and set it to be the Bevel Object of the Bezier Curve
The result seems to apply the Bevelling to the Bezier Curve before shrinkwrap, so the final curve object is flattened, not tubular:
I can instead Apply the Shrinkwrap before specifying a Bevel Object. This achieves the desired look, but is destructive (not easily modifiable):
This problem would be avoided if it were possible to specify when Curve Beveling should take place relative to the Modifier stack, but I haven't been able to find an option for doing so.
I've also tried using an explicit Bevel modifier on the Curve, since this would expose direct control over its placement in the Modifier Stack. This doesn't seem to be available for Bezier Curves in 2.79, but is an option for them in 2.80. I haven't found a Bevel Object option in the modifier, though, and so far haven't succeeded in making it change the Bevel Curve's shape visibly at all.