Here's the finished path extrusion script.  There are two GUIs, so it will run in both Poser 8+ (as wx) and Poser 7 and below (as Tk).  Capped ends, flattening along relative axes, and the option for a closed path have all been added.  The script no longer uses the scene callback and seems to be stable.

Basic instructions, also at the top of the script:

Use:

• Run the script.
• Press "Add marker" to create a path marker.
• Translate each marker as desired.  Place at path transition points.
• Place multiple markers (at least 3) close together to specify a sharper transition.
• Each marker will point at the previous one.  This is for display purposes only.  Marker rotation has no effect on the final path.
• Set "Spline type (0,1,2)" to define the type of spline used for the path.
      0 = NaturalCubicSpline ---> smooth path
      1 = Polyline ---> straight lines, sharp corners      
      2 = BlendedQuadraticSpline ---> similar to 0, with different handling of ends
• Set "Keep markers" to any non-zero value to retain the marker props at the end of a script run.
• Set "Close path" to any non-zero value to create a loop instead of an open-ended tube.
• "Flatten X" and "Flatten Z" can be set to non-zero values, to squash or stretch the tube along its relative X and/or Z axes.
• Negative values will squash the tube; positive values will stretch.
• "Flatten dot value" specifies the angle of inclusion for Flatten (as a dot product result).  0.0 = 180 degrees of inclusion; 1.0 = only vertices which are aligned with the axis.
• The "Flatten smooth reps" can be set to specify an amount of smoothing to apply to the flattening results.
• Examples:
      A broad, flat tube:
          Flatten Z = -0.3
          Flatten X = 2.0
          Flatten dot value = 0.9
          Flatten smooth reps = 3
      An indented tube, similar to an electrical cord:
          Sections = 20 (for adequate resolution)
          Flatten Z = -1.0
          Flatten X = 1.0
          Flatten dot value = 0.9
          Flatten smooth reps = 3  
• The other options work the same as those for the loop-making script.
• When the markers are placed, move the "RUN SCRIPT" dial.  The tube will be created.
• Move "END SCRIPT" to quit.

Notes:

• Some functions and classes below are adapted from various sources online.  See function notes for details.

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.