Certainly could be, I seem to remember I took a photo of a European treefrog at some point, must have been somewhere else .
In that case, I have no idea what species Doesn't look like all of the other Agalychnis lemur I saw yesterday though - seems too big (and also, only remember seeing them in the Nocturnal House and AmphibiPod - this is in the Reptile House in the small room off the main area (with a few exo-terras, and views into behind the scenes turtle area and breeding vivs)
Edit: Could this attachment be the European tree frog?
I would also think so. The eye is going vertically, and you can see the orange underneath. Other than a much duller orange, both of these are not usual characteristics of european tree frogs.