However, if you are able to pass along that result

I'm sorry, but don't understand what you're asking for. Your refactored code seems to do exactly what you've described--well done!

The only thing I can think that you might be asking for is to allow one to arbitrarily define the horizontal constraining line instead of hard-coding it. Maybe something like this (I've changed some of the parameters just because I liked the look better):

Manipulate[
 Graphics[
  {Darker@Green, Line[{{-constraint, 0}, {constraint, 0}}],
   GrayLevel[.5], Line[{-.5 length {Cos@angle, Sin@angle}, .5 length {Cos@angle, Sin@angle}}],
   Black, PointSize[.02], Point[{0, 0}],
   Blue, TranslationTransform[p][Line[{-.5 length {Cos@angle, Sin@angle}, .5 length {Cos@angle, Sin@angle}}]]},
  PlotRange -> {{-10, 10}, {-10, 10}},
  AspectRatio -> 1,
  Frame -> True],
 {{constraint, 5}, 0, 10},
 {{p, {0, 0}}, {-constraint, 0}, {constraint, 0}, Locator},
 {{angle, Pi/4}, 0, Pi},
 {{length, 10}, 1, 20}]

I'm not entirely sure I understand, but see if this is what you're looking for:

Manipulate[
 Graphics[
  {GrayLevel[.5], 
   Line[{-.5 length {Cos@angle, Sin@angle}, .5 length {Cos@angle, 
       Sin@angle}}],
   Black, Point[{0, 0}],
   Blue, 
   TranslationTransform[p][
    Line[{-.5 length {Cos@angle, Sin@angle}, .5 length {Cos@angle, 
        Sin@angle}}]]},
  PlotRange -> 10],
 {{p, {0, 0}}, {-5, 0}, {5, 0}, Locator},
 {{angle, Pi/4}, 0, Pi},
 {{length, 3}, 1, 5}]

This is almost identical to the previous, but we've constrained the locator, and you can see the difference where we specify the Locator control:

{{p, {0, 0}}, {-5, 0}, {5, 0}, Locator}

Maybe something like this:

Manipulate[
 Graphics[
  {GrayLevel[.5], 
   Line[{-.5 length {Cos@angle, Sin@angle}, .5 length {Cos@angle, 
       Sin@angle}}],
   Black, Point[{0, 0}],
   Blue, 
   TranslationTransform[p][
    Line[{-.5 length {Cos@angle, Sin@angle}, .5 length {Cos@angle, 
        Sin@angle}}]]}, PlotRange -> 10],
 {{p, {0, 0}}, Locator},
 {{angle, Pi/4}, 0, Pi},
 {{length, 3}, 1, 5}]