This depends on a few things. Where is your client in relation to the inpath subnet? Where is the default gateway pointing in relation to the client?
If the client is on the lan side of the inpath and on the same subnet, then the ping requests and responses will be handled on the lan interface.
However, if the client is on the wan side of the Steelhead and on a different L3 subnet and the Steelhead's default gateway is to the LAN side, then it will route the ping replies to the default gateway which will redirect them back.
So in the case where the Steelhead has to route the reply, it will use the default gateway and send that out of whichever interface that default gateway is located, arping for it if necessary.
