I was able to set up what you want with one caveat: You have to use a PIN or a password, not a pattern. My solution depends on the apps Taster and Secure Settings. Secure Settings requires root for some of its features. I don't know if this is one of them. If you aren't rooted, you can try it and see if it works.
Posting a complete how-to on Tasker is beyond the scope of my answer. Instead, here's the documentation. My solution involves setting up two Tasker profiles. The first one sets the PIN or password after a timeout. The second one clears the PIN/password once the screen is unlocked.
Here are my profiles:
Profile: Screen Locked (98)
Event: Display Off
Enter: Anon (106)
A1: Wait [ MS:0 Seconds:0 Minutes:5 Hours:0 Days:0 ]
A2: If [ %SCREEN ~ off ]
A3: Secure Settings [ Configuration:Set Pin Package:com.intangibleobject.securesettings.plugin Name:Secure Settings ]
A4: End If
Profile: Screen Unlocked (107)
Event: Display Unlocked
Enter: Anon (108)
A1: Secure Settings [ Configuration:Clear Password Package:com.intangibleobject.securesettings.plugin Name:Secure Settings ]
Once you've got Tasker configured, go to your phone settings and set slide unlock (and set he timeout to be whatever you find to be an appropriate amount of time for the slide lock to come up) . Note that when the password/PIN is enabled, you'll first have to slide unlock, and then enter your PIN/password.