Hi,
I don't think you will be easily able to implement a flow sensor using the LIFA firmware. Most code for reading these flow sensors use interrupts to detect the pulse outputs from the hall sensor. By measuring the time between these pulses, you can calculate flow rate. But I don't think the LIFA firmware has implemented hardware interrupts, so that wouldn't work.
A better solution would be to write a brief script on the Arduino to monitor the flow sensor with hardware interrupts, calculate the flow rate, and output the flow rate via serial to LabVIEW. Then LabVIEW can simply read the serial port through VISA, and output the data as you see fit.
My company (Biomated Solutions) provides automation systems to the biotech and pharmaceutical industry, and I actually implemented an Arduino/LabVIEW flow monitor for a client some time back. I should be able to find the code and send it over. And if not, it shouldn't take long to write it up again.
Best,
Vivek
Biomated Solutions LLC