I would take a look at the LL Snap example.  This example does the same thing as the High Level Snap Example, but does so using lower-level function calls.  This allows you to specify where you would like the buffer to be set up, in your case on-board.  Open the example and look at the function called imgCreateBuffer. This is where you specify that you want to use on-board memory.  The exmaple is located in "...\National Instruments\NI-IMAQ\Sample\MSVC\Snap\LL Snap" and the function is located on line 203.  Documentation for all of the c function can be found by going to Start >> Program Files >> National Instruments >> Vision >> Documentation >> NI-IMAQ Function Reference.