Easiest way is going to be to make the part defined in the assembly so the definition would update based on other parts.
One way: create the cross-section on one end, then define the opposite end in the assembly and dimension/relate it to the mating parts. Then if you loft between the two profiles SW will recreate the part when your assemly changes.
Alternate: do a sweep and have the guide curve defined in the assembly.
There are several ways depending on what is most realistic for your conditions.
SolidWorks doesn't really do physics-based deformation, so while it's easy in the real world to deflect a thin sheet or bend a wire those aren't easy things to accomplish in CAD.
Having said that, I would look into the following features:
Additionally, if you're adventurous, you can make your part using sheet metal features and then add in-context flanges, bends and jogs to make it match your assembly environment.
You can try doing a physics based deformation using Simulation Professional and then have SolidWorks display the deformed part.
You can make a dummy part showing the deformed condition and exclude it from the bill of materials in your assembly
Matt i would suggest you to subdivide your part,like many small rectangles,mate their edges; and later link the first and last one to the parts you want,then when those parts move,you will have a sort of flexible result
If you want to follow Chris's advice, I would suggest using Sweep rather than Loft. Loft is more likely to give you funny results. Bettery yet, you can extrude the part from the side with the S or L shape that you expect.
If you want to follow John's advice and try sheet metal, I would probably make your base flange in the S or L shape so that you don't have to add any bends or flanges or jogs. This is the way that I would probably go, as it is easy to use the flat pattern for the drawing of the part.