InDesign Server (via SOAP) -- Returning Binary Data: Possible or Not?

I think so. See ww-ids-soap.pdf in the SDK. You can return to the client any value you want; the value is typed in the SOAP response. An example from that pdf of a mixed-type array response:

<?xml version="1.0" encoding="UTF-8"?>
<SOAP-ENV:Envelope
    xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/"
    xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:xsd="http://www.w3.org/2001/XMLSchema"
    xmlns:IDSP="http://ns.adobe.com/InDesign/soap/">
    <SOAP-ENV:Body>
        <IDSP:RunScriptResponse>
            <errorNumber>0</errorNumber>
            <scriptResult>
                <data xsi:type="IDSP:List">
                    <item><data xsi:type="xsd:string">1</data></item>
                    <item><data xsi:type="xsd:string">2</data></item>
                    <item><data xsi:type="xsd:long">10</data></item>
                    <item><data xsi:type="xsd:long">12</data></item>
                </data>
            </scriptResult>
        </IDSP:RunScriptResponse>
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

Jeff