ColdFusion 10 Update 11 404 handler not firing

ColdFusion 10 Update 11 404 handler not firing

I know CF 10 has a number of issues surrounding 404 handling. This seems
to be different from the other reports. Details:
Win2k8 R2/64 and IIS7.5
Upgrading from identical config on separate server. Only difference is CF9
-> CF 10. All works fine on CF9. Adobe CF9 Lockdown implemented on
original server, CF10 Lockdown implemented on this server.
missing template handler set in CF Admin as /404.cfm, which should
translate to the Cfusion root (c:\ColdFusion10\cfusion\wwwroot).
IIS has been config'd to trace failed 404 requests
IIS 404 handling is default (originally executed a CF URL but removed to
simplify debug).
Coldfusion webroot where missing template handler resides is default
install location
IIS site root is entirely different: c:\Other\Place\SiteRoot\
A sitewide error handler is also set in CF Admin in the same ColdFusion
webroot and works as expected.
404.cfm is very simple:
<cfheader
statuscode="404"
statustext="Not Found">
<h1>404</h1><p>Page not found</p>
<cfoutput>#now()#</cfoutput>
Inputting the bad url [domain]/foo.cfm should display the above template.
Instead I get an IIS error screen. The CF missing template handler is
ignored. The IIS failed request trace says the url is
http://[mydomain]:80/jakarta/isapi_redirect.dll
and a detail view shows at Step 1
RequestURL="http://[mydomain]:80/foo.cfm
I've seen plenty of issues surrounding CF10 and 404's, but never that the
missing template handler assignment is completely ignored. In CF9 this
will generate output as expected. Anyone seen anything like this?
EDIT: I have also tried config'ing this to match a different CF9 server I
have running: Added a CF mapping to the web root of the site. Then placed
the missing template handler in the web site's root rather than the CF
default web root, lastly in cfadmin pointed to the missing template
handler in the web site's root using the mapped folder. Same problem.
Works fine in CF9 and not at all using CF10.