Well, after a nearly a year of letting this question marinade, I gave it another shot. Here’s the web.config magic that got me what I wanted:
<!-- inside of <configuration> to allow error
responses from requests to /api through -->
<location path="api">
<system.webServer>
<validation validateIntegratedModeConfiguration="false" />
<httpErrors errorMode="DetailedLocalOnly" existingResponse="PassThrough" >
<clear/>
</httpErrors>
</system.webServer>
</location>
<!-- original httpErrors, inside of <location path="." inheritInChildApplications="false">
to serve custom error pages when the MVC site returns an error code -->
<httpErrors errorMode="Custom" existingResponse="Replace">
<remove statusCode="400" subStatusCode="-1" />
<remove statusCode="404" subStatusCode="-1" />
<remove statusCode="500" subStatusCode="-1" />
<remove statusCode="403" subStatusCode="-1" />
<error statusCode="400" prefixLanguageFilePath="" path="Content\notfound.htm" responseMode="File"/>
<error prefixLanguageFilePath="" statusCode="404" path="Content\notfound.htm" responseMode="File" />
<error statusCode="500" path="/errors" responseMode="ExecuteURL" />
<error statusCode="403" path="/errors/http403" responseMode="ExecuteURL" />
</httpErrors>
The crux of what’s going on here is that the <location> node allows you to override settings made at a less specific path. So while we have errorMode=”Custom” for path=”.”, we override that for the our Web API’s path with the <location path="api"> node, and the httpErrors configuration within it.
I had seen nodes before, but it didn’t dawn on me that this was their purpose until now. This article goes into more detail on the configuration inheritance model of IIS/.NET, which I found very informative: http://weblogs.asp.net/jongalloway/10-things-asp-net-developers-should-know-about-web-config-inheritance-and-overrides