Hello and welcome to the forums!
This can probably be done a number of ways depending on what you need:
- How are you intending to show the fixture in Plan View? Were you aiming to show the actual geometry of the fixture, or just a Symbolic Line representation?
- How details will the geometry of this Family be? It would perhaps be best to model the body of the Fixture separately and load it into a host Family which will control its angle.
- Even with normal suspended fixtures, it is really no very possible to set it up to specify the elevation from the floor unless the Family is Level-Based, in lieu of hosting to the ceiling face. In the case of the latter, you could specify the suspension length for each side.
- As a tip, I created something like this a while ago, and I included Invisible Lines to indicate which side was which when adjusting the angles from a Plan View. These indicator lines appeared only when the element was selected.
Corey D. 
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In this case, the solution should be pretty easy. Since you're going to represent the fixture body with a single Extrusion, you won't need a Nested Family. You can use a Sweep Extrusion and use Reference Planes to control the angle. I'll briefly explain my steps for a simple solution, starting with a blank Lighting Fixture family.
1. Create two Reference Planes; one to determine the height of each side of the fixture, create a Parameter for each, and then constrain with a Dimensional Constraint.
2. Create two vertical Reference Planes if you want the center of the Fixture as the Base Point or one Reference Plane if you want one end to be the Base Point.
3. Create the Parameters to control the Fixture Body's Length, and Height and Width (if desired). Now, the horizontal span of the fixture will not be equal to the Fixture's specified Length if it is hung at an angle. Create another Length Parameter to constrain the horizontal Length. Use the formula:
= sqrt(FixtureLength ^ 2 - abs(SuspLength1 - SuspLength2) ^ 2) (using pythagorean theorem)
4. Create a Sweep Extrusion, and draw the Path between the two appropriate intersections of Reference Planes. They should constrain themselves. As a tip, if you are ever drawing a Path at an angle, draw the first segment as a small, level line so it will be easier to draw the Profile. When completed, go back and delete the extraneous Path line.
5. Create the Sweep's Profile and apply the Fixture's Dimensional Parameters as Constraints, if desired.
6. Model the cables, if desired. Models Lines work well for simple cables.
Again, this is very brief so don't hesitate to ask questions. Attached is a sample Family file (Revit 2013). Try creating your own - don't just use the example file!
Corey D.
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As an edit to my previous post, use the formula below instead:
= sqrt(abs(FixtureLength ^ 2 - (SuspLength1 - SuspLength2) ^ 2))
The "abs" function means Absolute Value. Since the SuspLenths are squared, it's impossible to have a negative value there (which would produce an error). Instead, the "abs" should apply to the entire equation prior to the Square Root function in order to avoid a potentially erroneous negative value (since there is no square root of a negative number).
Corey D.
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Accepted solution
levitd wrote:"I think I've gotten everything up to making the sweep. So when I create the sweep's path, I should pick the intersection of the SuspLength plane and the two vertical planes defining Horizontal Length (the calculated value)? Is that correct?"
That's correct. By drawing the Line from one intersection of Reference Planes to the next, it should automatically associate with them.
levitd wrote:"Once I've created the sweep path and am editing the profile, how do I actually constrain the dimensions (height/width) to parameters? I have to have already created reference planes w/dimension constraints for each of those, and then just draw the profile as a rectangular intersection of those four planes (top, bottom, left, right-sides of fixture), right? For some reason, I can't get that to work for me."
You do not necessarily need Reference Planes for each and every constraint in a Family; you can instead just apply Dimensional Constraints to the Profile's Sketch Lines while in Sketch Mode. Open the sample Family and edit the Profile and you'll see my "internal" constraints within the sketch. It will appear kinda wonky if it's at an angle, so level out the fixture before looking.
This is why I typically draw a small section of "level" Sketch Path (as described in my previous post); for the purposes of creating and managing the Profile Sketch. OR, you can simple open the Family Types dialog while in Sketch Mode and even out the fixture.
levitd wrote:"I think I could get it if I didn't want to be able to change the width/height, as I could just model it as the exact fixture I'll be using, but obviously it would be better if I could re-use this family. I may just use the one you uploaded until I can get this figured out, if you don't mind!"
I don't mind, but it sounds like you're close!
Corey D.
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