Specifying alternate error log directory via JAVA_OPTS

I'm upgrading from SoapUI 5.4.0 to 5.7.0 and trying to put the log files in a specific directory. I was able to do this in 5.4.0 but can't get the config for 5.7.0.

I have the following specified in my JAVA_OPTS for SoapUITestCaseRunner:

-Dsoapui.logroot="%SOAPUI_LOGSDIR%"
-Dsoapui.log4j.config="%SOAPUI_HOME%/soapui-log4j.xml"

In my soapui-log4j.xml I specify the error file as:

<RollingFile name="ERRORFILE"
     fileName="${soapui.logroot}/soapui-errors.log"
     filePattern="${soapui.logroot}/soapui-errors.log.%i"
     append="true">

The error file then gets created but without resolving ${soapui.logroot} e.g.

$ find . -name "*errors*"
./${soapui.logroot}/soapui-errors.log

I also tried specifying it as lookup but ended up with this:

ERROR Unable to create file ${sys:soapui.logroot}/soapui-errors.log java.io.IOException: The filename, directory name, or volume label syntax is incorrect 

Any suggestions for my next step?