I've had trouble posting python scenes to members with older versions recently, so a description will have to do.
In this scene I used GetTags() to get a list of tags from the object, then tags to reference the actual tag by index. For the correct syntax for things like sliders, I find just dragging the field into the Python window works (with a bit of editing). Not sure if this works with your R15.
global Output1 # for test
length = Input1 # constraint length
cube = doc.SearchObject('Cube') # obj with tag
tags = cube.GetTags() # get all tags on obj
constraint_tag = tags # get 1st tag (or whatever)
# drag field from attrib manager to get syntax
# may need some editing
constraint_tag[50004,9] = length # set constraint length
Output1 = length # test