Is there a way to determine what ProjectWise Client external users are connecting to a Datasource with?

hmmm. I'm not sure it has all the data you're looking for but if in dmskrnl.log.xml you bump up both the pwise.connection and pwise.dms.login to the info level you will get reports on both the users logging in and the client they are using (and version). I'm not sure what the web clients report. You can tie the login user info with the client connection info together via the thread id in brackets on the log line.

If you really want to make this data parsable you can push it into it's own log file. Edit the dmskrnl.log.xml, duplicate the RollingFileAppender section, give it a new name and new log file. Then modify the pwise.dms.login and pwise.connection to send their logs to the new appender.

<appender name="LoginRollingFileAppender" class="org.apache.log4j.RollingFileAppender">
        <param name="File" value="C:\Logs\Logins.log"/> <!-- Required -->
        <param name="MaxFileSize" value="10000000" />                   <!-- Required -->
        <param name="MaxBackupIndex" value="3"/>                       <!-- Required -->
        <param name="CreatePath" value="true"/>                         <!-- Optional -->
        <!-- param name="encoding" value="UTF-16" -->                   <!-- Optional -->
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%d{ISO8601} %-5p [%t] %c - %m%n"/>
        </layout>
</appender>

    <!-- Messages related to connections to the server. -->
    <category name="pwise.connection">
		<priority value="info"/>
		<appender-ref ref="LoginRollingFileAppender"/>
	</category>

    <category name="pwise.dms.login">
		<priority value="info"/>
		<appender-ref ref="LoginRollingFileAppender"/>		
	</category>

Usage reports on reporting.bentley.com would also provide insight into what client users are using to connect to the datasource. Each of those applications, PW Web, V8i Web, and Explorer, will all have different IDs.