How do I redirect a parameter with an underscore in tuckey url rewrite?

Question

I need to redirect old URLs in the form:

/123?var_artikel=666666 to the new URL: /en-GB/product-666666.html.

I tried this rule:

<rule match-type="regex">
    <condition type="parameter" name="var_artikel" operator="equal">^([0-9]+)$</condition>
    <from>^/123?</from>
    <to type="permanent-redirect" last="true">/en-GB/product-%{parameter:var_artikel}.html</to>
</rule>

However the browser tries to redirect to /en-GB/product-%%7Bparameter:var%5C_artikel%7D.html

If I remove the underscore in {parameter:var_artikel} it works. So the underscore is the reason it doesn't work, but of course I need it. Is there a way to escape this underscore? Escaping with \ didn't work..

Edit: Found it, seems to be a bug in Tuckey 4.0.3+

https://code.google.com/p/urlrewritefilter/issues/detail?id=157&colspec=ID%20Type%20Status%20Summary%20Stars

Anyone got a workaround?