Following the link in Duopixel’s comment, the problem can be solved using a mask:
<svg width="50%" height="50%" viewbox="0 0 985 740" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns="http://www.w3.org/2000/svg">
<defs>
<rect id="sa11" x="763.0" y="176.5" width="70.0" height="25.0" rx="50" ry="50" />
<rect id="sa12" x="516.0" y="127.5" width="70.0" height="25.0" rx="50" ry="50" />
</defs>
<mask id="re8-clip">
<rect id="bg" x="0" y="0" width="100%" height="100%" fill="white"/>
<use xlink:href="#sa11" fill="Black" />
<use xlink:href="#sa12" fill="Black" />
</mask>
<use xlink:href="#sa11" fill="ForestGreen" />
<use xlink:href="#sa12" fill="ForestGreen" />
<path stroke="Black" stroke-width="1.5" fill="none" d="M 798.0 189.0 551.0 140.0" mask="url(#re8-clip)"/>
</svg>As a minor aside, does anybody know if it is possible for a mask to default to white, so the ‘bg’ rectangle is not necessary?