Set HTTP response code on custom endpoint

I'm not entirely sure it's possible to change the status code to something other than 200, but what I do to still report any unsuccessful outcomes in my custom endpoints is to define "status" and "message" values (similar to v2 responses) and return them in JSON format.  Then I just use FreeMarker (or Python or whatever if not directly within Khoros) to parse the JSON response.

For example:

<@compress>
<#-- Define default variables -->
<#assign status = "error" />
<#assign message = "" />

<#attempt>
  <#-- Perform my endpoint operations -->
  <#assign status = "success" />
  <#assign message = "Operation completed successfully" />
<#recover>
  <#assign message = .error?json_string />
</#attempt>

<#-- Define the JSON data to return -->
</@compress>
{
  "status": "${status}",
  "message": "${message}"
}

Hope this helps!