Ah and here I was thinking to just 'stop' drawing when the static length is reached, and let the user fiddle until it 'fits' 
I don't know if the setup I currently have in mind will give you a way to test your work efficiently (unless I add scripting). My current plan is:
Create a submodel with some specialized header meta's (e.g. 0 !LDCAD GENERATED PART).
Add a bunch of additional meta's describing the general path.
Then in a/the model referencing this special submodel, you can manipulate the path of the part using the mouse etc much like normal placement of parts.
When ever the path changes LDraw content for the special submodel will be regenerated.
If you have any ideas on how to improve this so it might help your intentions better, let me know.
I don't know if the setup I currently have in mind will give you a way to test your work efficiently (unless I add scripting). My current plan is:
Create a submodel with some specialized header meta's (e.g. 0 !LDCAD GENERATED PART).
Add a bunch of additional meta's describing the general path.
Then in a/the model referencing this special submodel, you can manipulate the path of the part using the mouse etc much like normal placement of parts.
When ever the path changes LDraw content for the special submodel will be regenerated.
If you have any ideas on how to improve this so it might help your intentions better, let me know.