Move vertex along spline in editable spline object

Hello and thanks everyone who can helps
How move vertex along spline in edit spline object and don't change contour of spline? meybe something similar to edit poly objects - moving vertex constraints edge mode?
Or even much better put distance between vertex but it mustn't change spline's contour

and example what really needed:

P.S. And I know this script
have some function - Constrain Spl. Verts, but it doesn't work good enough