Hi buke,
L2PT is not supported on QFX5100. There you need til ACX5000 series to handle this.
Regarding EX4300 it should be supported when looking in the feature explorer for L2PT: https://pathfinder.juniper.net/feature-explorer/feature-info.html?fKey=1133&fn=Layer+2+protocol+tunneling+(L2PT) - also note that QFX5100 is missing from this list.