Hi CAD-One,
Many thanks for submitting your query. Unfortunately, Boundary Patch does not currently support guide rail definitions - only the boundary geometry and tangency conditions of the surface can be defined. However, we do understand that being able to define additional geometry constraints would offer some useful functionality for surfacing workflows, and I am forwarding your post to our management team as further evidence of this; this will be taken into consideration for future Inventor development plans.
For now, perhaps it’s possible to create your model with two or more connected surfaces (Lofts or Patches), using G2 constraints to ensure a smooth transition between them? If you find that this approach doesn't seem to be working for your design, I would be more than happy to take a look at your model and offer some support (this would work best if you are able to share the dataset: you could upload the model to the forum, or alternatively you can email it to me directly at jake [dot] fowler [at] autodesk [dot] com).
Look forward to hearing back from you,
Jake
Jake Fowler
Principal Experience Designer
Fusion 360
Autodesk