In order to do custom authentication you will need to recompile Apache with the auth_external module included. The auth_external source comes with Apache but by default it is not included in the compiled version.

Don't know about you, but for us the need to recompile Apache (on an old, no-longer-supported version of NCR Unix) was a show-stopper. We told the client that we could not do what they wanted without recompiling, and they said forget about it then.