largely based on an experiment that I found, here's what I've come up with:

<defs><filter id="inset-shadow">
    <feOffset dx="10" dy="10"/>                                                         <!-- Shadow Offset -->
    <feGaussianBlur stdDeviation="10"  result="offset-blur"/>                           <!-- Shadow Blur -->
    <feComposite operator="out" in="SourceGraphic" in2="offset-blur" result="inverse"/> <!-- Invert the drop shadow to create an inner shadow -->
    <feFlood flood-color="black" flood-opacity="1" result="color"/>                     <!-- Color & Opacity -->
    <feComposite operator="in" in="color" in2="inverse" result="shadow"/>               <!-- Clip color inside shadow -->
    <feComponentTransfer in="shadow" result="shadow">                                   <!-- Shadow Opacity -->
        <feFuncA type="linear" slope=".75"/>
    </feComponentTransfer>
    <!--<feComposite operator="over" in="shadow" in2="SourceGraphic"/>-->                       <!-- Put shadow over original object -->
</filter></defs>

<rect width="100" height="100" fill="yellow" filter="url(#inset-shadow)"/>

If you want the fill to be seen, uncomment the last <feComposite>. Unfortunately, fill="transparent" will not give the filter an alpha to work with and will produce no shadow.

largely based on an experiment that I found, here's what I've come up with:

<defs><filter id="inset-shadow">
    <feOffset dx="10" dy="10"/>                                                         <!-- Shadow Offset -->
    <feGaussianBlur stdDeviation="10"  result="offset-blur"/>                           <!-- Shadow Blur -->
    <feComposite operator="out" in="SourceGraphic" in2="offset-blur" result="inverse"/> <!-- Invert the drop shadow to create an inner shadow -->
    <feFlood flood-color="black" flood-opacity="1" result="color"/>                     <!-- Color & Opacity -->
    <feComposite operator="in" in="color" in2="inverse" result="shadow"/>               <!-- Clip color inside shadow -->
    <feComponentTransfer in="shadow" result="shadow">                                   <!-- Shadow Opacity -->
        <feFuncA type="linear" slope=".75"/>
    </feComponentTransfer>
    <!--<feComposite operator="over" in="shadow" in2="SourceGraphic"/>-->                       <!-- Put shadow over original object -->
</filter></defs>

<rect width="100" height="100" fill="yellow" filter="url(#inset-shadow)"/>

If you want the fill to be seen, uncomment the last <feComposite>. Unfortunately, fill="transparent" will not give the filter an alpha to work with and will produce no shadow.