Off-load voltage of wall-warts
Well, without having much information about what constitutes a minimum load for regulation, here's a quick and dirty way to get what you want done:







simulate this circuit &ndash; Schematic created using CircuitLab

Now, as soon as the wall-wart is attached, you are at least loaded with the LED current ( higher current = better regulation, but lower efficiency of design) + the 7805 current (~5 mA internal + 5 mA for R1). I'd think this should be more than enough to get any wall-wart into its regulated 12 V, plus you have an LED to show when power is applied.