You could use a filter to generate the background.
<svg width="100%" height="100%">
<defs>
<filter x="0" y="0" width="1" height="1" id="solid">
<feFlood flood-color="yellow" result="bg" />
<feMerge>
<feMergeNode in="bg"/>
<feMergeNode in="SourceGraphic"/>
</feMerge>
</filter>
</defs>
<text filter="url(#solid)" x="20" y="50" font-size="50">solid background</text>
</svg>