[SalesForce] Apply filter for lookup on custom page

You should not reference a Sobject, reference its field instead. Correct way is:

public Account contactLookup { get; set; }

<apex:inputField value="{!contactLookup.contact}" />

<apex:inputField> binds with sObject's field not the whole sObject. If it is a lookup then it will have lookup style.

Per Jeff Douglas's ROLL YOUR OWN SALESFORCE "LOOKUP" POPUP WINDOW blog post, one way to do this is to simply create a dummy instance of an object that has a relationship with the one you want to perform a lookup on, and then use the appropriate property of that instance as the value of an inputField.

From his post:

MyCustomLookupController

Here's the Apex controller for the record you are either creating or editing. This is an extremely simple controller that just creates a new contact so you can use the lookup for the related account field.

public with sharing class MyCustomLookupController {

  public Contact contact {get;set;}

  public MyCustomLookupController() {
    contact = new Contact();
  }

}

MyCustomLookup

[ ... lots of code omitted; see the original post for full content ... ]

<apex:inputField id="Account" value="{!contact.AccountId}"/>

This is an ugly, hacky solution, and I'd hoped to find something cleaner, but it works. It'd be even uglier if you didn't already have an object with a field of the right type; in that case, I guess you could create a custom object whose entire purpose was to be used for this hack.