I found this site for ...engineautolube...I've seen engine prelubers on tractors before. This might help if the piston slap could be dealt with by getting more oil to the piston skirts before startup. I don't think it will help if the piston needs to get a little heat into it so it expands and ends the piston slap.
What do you guys think? I see this one is electric which is nice. $200.00 isn't bad for what it does. Many others require air.

 

Since the skirts and cylinder walls get oil that is slung off the crank, I doubt that a pre-luber would do much for the piston slap problem. Changing to a synthetic oil may help as it has a tendency to cling to parts better than conventional oils.