Here are some "secret" INI keys related to CLFNs:
extFuncGenNoWrapperEnabled=True - the most interesting one here; adds "Generate Wrapper" option to CLFN context menu (on by default). When this option is unchecked (off), LabVIEW doesn't generate the wrapper for DLL call, i.e. ExtFuncWrapper function is not called and the user's function is inlined into the VI code and called. This feature could slightly improve performance of external code communications, saving approx. 5 to 10 ms on each call. But you must be absolutely sure, that your CLFN is set correctly as no error checking is done in this case and any small mistake in the parameters leads to LV crash. There might be an indirect use of this feature, e.g. manual fine-tuning/debugging of some problematic function or a function with unclear parameters etc. When the option is enabled and extFuncShowMagic is set, CLFN is painted red.
extFuncBreakOnGenNoWrapper=True - if enabled, disallows to run VI with CLFN's "Generate Wrapper" option unchecked. The Run arrow is broken and the error is stated as "Call Library Nodes with 'gen no wrapper' are not supported.". The error is detailed as "This Call Library Node is set to 'gen no wrapper'. This setting skips certain steps when calling DLLs in order to be faster, and can be dangerous.".
extFuncShowMagic=True - adds some coloring to CLFNs, when "Generate Wrapper" and "Type Only Argument" options are used.
extFuncExtendedOptionsEnabled=True - adds "Allow Sub-Arrays" option to CLFN context menu (off by default). I assume, it should have worked this way, but I have not found any signs that this feature is working at all. If I set "Constant" option, then a sub-array is passed into a function, else the whole array is copied and passed, no matter if "Allow Sub-Arrays" is set or not. Maybe I missed smth or it's a relic option from pre-LV2009 era...
extFuncArgTypeOnlyEnabled=True - adds "Type Only Argument" option to CLFN context menu, when clicking on a node's terminal (off by default). When this option is checked, LabVIEW passes 0 (aka NULL) instead of a real value of the parameter, no matter if passed by value or by reference. When the option is enabled and extFuncShowMagic is set, the terminal is painted bluegreen (cyan). Any thoughts, what was it used for? Here's its scripting counterpart.
allowPreallocsInDll=True - adds "Allow Preallocated Data" option to CLFN context menu, when clicking on a node's terminal (off by default). Did not study this one well, but it seems to not do anything or I'm wrong? Here's its scripting counterpart.
callLibNodeMayHaveLVClassParams=True - if enabled, allows to run VI with LV class wires passed into CLFNs. By default the Run arrow is broken, when class wire is connected to CLFN. Interesting, but what can we do with it in the external code?
AllowCLFNTakeSlice=True - unexplored too atm.
This is the pic with some options activated:
I also noticed that there are some private flag bits for CLFNs, e.g. extFuncVarArgs, extFuncParamsFromTL, extFuncNIValidated, extFuncTopLevelOnlyInUI.
I believe, some bits should be available with scripting also. Var args feature looks interesting, but it's likely an abandoned thing, isn't it? AFAIK CLFNs never supported that in any manner.
Finally some sort of off-topic, but somewhat related.
Did you know that it's possible to create an input constant for CLFN's return terminal?
Just set Return output as non-void and do a RMB click on the left terminal. The Run arrow is not broken and the VI runs fine. Ancient bug or intended for something? I've never seen that value being passed into external code in any ways.