Back when I used to use seafoam (before I discovered FP60 by
Lube Control) I always let mine stall.
Seafoam has such high volatility and low flash point I highly doubt you'd hydrolock the engine unless you used a significant amount. I only used 1 oz per cylinder (so 8 oz on my V8, 4 oz on my 4 banger).
If there was a risk you'd hear about it on the internet all the time.
Besides, one of the two ways Seafoam works is by solvency. You want it in liquid foam on the piston.