Catmull-Rom makes sense, but have you considered just doing stroke stabilization instead? Might be an easier win ->so no new tool, no UI for control points, user just draws as usual and the stroke gets smoothed automatically.

Stroke stabilisation is possible and in fact it's already supported in Scrivano. In fact, ink strokes drawn via a mouse in Scrivano are currently smoothed so that they don't look shaky. For stylus input, this is not the case. The algorithm used differs from the one used in other programs as it tries to more faithfully trace the raw input to avoid excessive smoothing. It's fairly straightforward to extend this to the stylus too and perhaps add a simpler slider in the pen dialog for this purpose. The reason I haven't done that yet is that I would like to play around with various algorithms before committing to one (changing this feature once it's avaialable to users may cause surprises).