In IIS 7, the 301 Redirect system in mojoPortal current release (2.3.0.4.b) "may" work for .html files if you edit the system.webServer modules section
add the empty preConditon to the UrlRewriter module like this:
<add name="UrlRewriter" type="mojoPortal.Web.UrlRewriter, mojoPortal.Web" preCondition="" />
and add this attribute onto the <modules> element like this: <modules runAllManagedModulesForAllRequests="true">
It definitely works in the latest svn trunk so it will work in the next release.
Then you can create mappings under Admin > Advanced Tools > 301 Redirect Manager
that map somefile.html to somefile.aspx
Hope it helps,
Joe