It really seems that defining the error code is a bit harder than it should be.
However, one workaround MIGHT be using the mod_rewrite. Popped that one out of my head due the fact that in at least 99% of impossible-looking Apache problems mod_rewrite can provide its voodoo dance and solve your problem.
If everything else fails, try something like
RewriteEngine On
RewriteCond %{REQUEST_METHOD} !^GET [AND]
RewriteCond %{REQUEST_METHOD} !^POST
RewriteRule .* /yourerrordocuments/405.html [R=405,L]
I know that is probably not the nicest solution around, but it should kind of work...