See in a basic servlet api there are 3 main scopes, request, session and application (ServletContext).
Struts "ActionContext" getContextMap/parameter gives a copy of context-params specified in web.xml.
These values are ment to be readonly once application is deployed, as per servlet api.
Hence, it did not worked out. And yes its a bug in Struts that ActionContext.getContextMap gives you a Map instance which is modifiable.