AutoCAD Inventor :: ILogic Part Snap To Sketch For Walkways?

The company I'm working for is moving toward standardising all plant designs.  iLogic appears to be the best way for rapidly producing assemblies that keep to a standard. 

My new task is to automate the adding of walkways (including hand rails) to large plants.  My idea is that walkways would be added to the plant similar to how frame generator works.  The draftsman would draw a sketch on a plane around the structure.  He would then run a form (That looks like the frame generator form).He would select the lines, a rule would capture the length of that line, pattern the standard walkway along the line until the line intersection, add a corner walkway, and then pattern the straight walkways again.

1) I do not know the ilogic code in order to 'get' a length from the lines that the user selects.

2) how does one constrain the center line of a part to a sketch line.  
 
The second idea I had to create the walkway is (without a sketch), was to have a form that has a dropdown list of all the standard walkway pieces.  The form would work almost like adding a bolted connection to an assembly (e.g. bolt, washer, nut, washer, nut).  The draftsman would for instance,  choose an straight 800mm part, a 1200mm part, a corner(left), a 1200mm part.  Each walkway assembly piece, would then be added to the main assembly in the correct order and constrained.  BTW, is there any book with all the inventor iLogic code, function and uses?  I have found myself pouring though so many forums and picking up bits and pieces, but no actual instruction.