To clarify, your espresso machine should already have an expansion valve (*) that allows a few drops of water to escape from the group hydraulics into the driptray during warmup. They're typically set to crack open at 12 bar so the pump's brew pressure won't open the expansion valve. The inlet backflow preventer is one-way so the increase in the espresso machine's hydraulics system doesn't push water back into the inlet line; it also prevents water from being sucked out of the machine if there is a failure in water pressure from the main.

That said, since the machine's hydraulics are designed for high pressure and the expansion valve acts as an escape valve, I don't understand your concern about pump seals.

(*) Also referred to as an over-pressure valve (OPV) when speaking of espresso machines with vibratory pumps.