Indeed. That requires modifications to Harmattan operating system files, which is something applications should not do for various reasons. Those willing to make (potentially dangerous) modifications to their system are of course free to do so and for that purpose QuickTweak is probably a suitable tool, but users of such modifications need to be aware of the risks and consequences.

So, in short: The lock screen is not a part of the home screen and by that reason alone is beyond the scope of Home Screen Settings. Also, this requires modifications to system files, which is not allowed in Store applications (and is also something a "consumer" application should not do).