Mode Modifiers??? Something like that. ALMOST needs to be a generic widget, instead of even a keyboard thing...
Cover labeling of the modifier key and the individual keys: show the correct mode for virtual keypads, never caps labels when in lower-case, put the shifted label above it and gray if needed, as on a hardware keypad, etc. Make sure to keep consistent: avoid having one lock key and the rest use press-twice, but if you do then make sure there's a reason like access to the fn number pad is needed sometimes; never switch modes between virtual and hardware keyboards, or when you switch between keypad and keyboard, etc.
Problem
Solution
Variations
Toggles: Dedicated key, with sub-modes Press and hold, press-twice, or dedicated secondary lock keys.
Not-toggles (need a name): Like triple taps mode selectors
Both work on virtual and hardware keyboards and keypads, and can be used as the mode selector for pen input methods as well. The same principles may also be applied to any mode switch, but will be discussed here as they relate to supporting character entry.
Interaction Details
Always sticky. In the sense of accessibility keypads. You never have to hold a key down, to get a modifier. So simple shift moves from being a modal modifier (on a desktop) to a single input modifier, and lock is needed for multiple key input.
Multiple lock modes... In variations?
Presentation Details
Indicate locked. For virtual, change the symbol AND change the keycaps to reflect it... For hardware, generally on-screen, see the mode indicator pattern
Antipatterns