After some tests using the vanilla VR template (4.13) I noticed that calling the “Reset Orientation and Position” node do reset the orientation and position of the HMD for the whole unreal editor session (ie. between two playtesting). Restarting the editor look like the only way to come back to the default value.
Since I use multiple pawns with specific behaviors (one of them is using this node) I think that having a way to switch back to the original (SteamVR) origin would be useful (either that or the ability to store and set the origin manually).