THe getSlingScriptHelper().getService();
only works from WCMUsePojo.
I just tested using @inject and it works to inject a running AEM Service into a Sling Model .
Given these classes:

KeyService interface:
package com.community.aem.core;
public interface KeyService {
public void setKey(int val);
public String getKey();
}
KeyServiceImpl class
package com.community.aem.core;
import org.apache.felix.scr.annotations.Component;
import org.apache.felix.scr.annotations.Service;
//This is a component so it can provide or consume services
@Component
@Service
public class KeyServiceImpl implements KeyService {
//Define a class member named key
private int key = 0 ;
@Override
//A basic setter method that sets key
public void setKey(int val)
{
//Set the key class member
this.key = val ;
}
@Override
//A basic getter that gets key
public String getKey()
{
//return the value of the key class member
//Convert the int to a String to display it within an AEM web page
String strI = Integer.toString(this.key);
return strI;
}
}
.
We can inject KeyService into HelloWorldModel - see:
package com.community.aem.core.models;
import javax.annotation.PostConstruct;
import javax.inject.Inject;
import javax.inject.Named;
import com.community.aem.core.KeyService;
import org.apache.sling.api.resource.Resource;
import org.apache.sling.models.annotations.Default;
import org.apache.sling.models.annotations.Model;
import org.apache.sling.settings.SlingSettingsService;
@Model(adaptables=Resource.class)
public class HelloWorldModel {
@Inject
private KeyService keyService;
@Inject
private SlingSettingsService settings;
@Inject @Named("sling:resourceType") @Default(values="No resourceType")
protected String resourceType;
private String message;
@PostConstruct
protected void init() {
keyService.setKey(80) ;
message = "\tHello World! - the keyservice is " +keyService.getKey() +" \n";
message += "\tThis is instance: " + settings.getSlingId() + "\n";
message += "\tResource type is: " + resourceType + "\n";
}
public String getMessage() {
return message;
}
}
This works -- you can see the successful output here. I hope this clears up how to reference a running AEM Service from WCMUsePojo and Sling models.

You can try this too by following this article to get the default class:
Creating an Adobe Experience Manager 6.3 Project using Adobe Maven Archetype 11
Then add the KeyService and Impl class to the com.community.aem.core package. You will get the same result. BE sure to add the new code (bolded code above) to the Sling Model class too!