From what I can guess, you need to call a register function before you can call from that group of functions. Is this a property of CHDK itself or the Canon firmware?
It's entirely defined by the Canon firmware. The eventprocs are available by name from Canon's own built in scripting language (
http://chdk.wikia.com/wiki/Canon_Basic), and some of those eventprocs register other eventprocs. I made a fairly comprehensive list for a540
http://chdk.wikia.com/wiki/User:ReyalP/EventProcNotes but it's an old vxworks camera (without IS) so will not be completely right for newer cams.
If you have the address, then you can call it directly. However, you may have to adjust how the arguments are past. For example on many cameras if you call the address of PT_PlaySound with call_func_ptr, it will expect a single argument which is a pointer to the two real arguments, while if you call it as an call_event_proc, you must pass 2 individual and the canon enventproc system will automatically convert it (note the _PT_PlaySound in CHDK stubs will point to a lower level function on these cameras)
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It's also worth noting that some of the registration eventprocs also do other things, so calling the functions directly may skip some expected initialization.
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On my D10, many IS releated functions appear to be registered by StartImStEventProc, which itself is registered by Mecha.Create. The first function registered by this is "ISInitMotion"
MoveISLensTo* ect looks like they should be registered by InitializeAdjustmentSystem