Are there any specific planning techniques that are used to avoid underdosing the match line in a single isocenter breast plan treating the chest wall and supraclavicular region?  

I tend to use a high single iso match when possible to limit lung dose through the AP SCV. What we do is open up a segment from the SCV into the tangent to heat up the match and avoid underdosing of the axilla. Well seen in Figure 6.16 in this treatment planning book.