You can use clip-path to clip to whatever shape you want, see e.g masking-path-01 from the svg testsuite.
Relevant parts, defining the clip path:
<clipPath id="clip1">
<rect x="200" y="10" width="60" height="100"/>
... you can have any shapes you want here ...
</clipPath>
and then apply the clip path like this:
<g clip-path="url(#clip1)">
... your text elements here ...
</g>