You can opt to just put the filename in the Attributes collection property of the custom tracelistener. Your custom tracelistener now should have the constructor with the NameValueCollection parameter.
public CustomTraceListener(NameValueCollection attributes)
fileName = attributes["fileName"];
The "fileName" key is based from the Key you used for the value of filename in the Attributes collection property.
The other way would be to modify the entlib source code so as to create design time support for your custom tracelistener which is so much more tedious as compared to the first suggestion.
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