Apart from a reference in the 1974 Pilotenzuteilunglist, which defines it as an alternate of the H-5 on Mirage IIIS - to our knowledge there are no official documents about a possible evaluation/ entry into service of the HGU-2 in the Flugwaffe’s jet aircraft fleets.
Nevertheless, here a single visor HGU-2A/P sporting a "Tiger" cover decoration, fitted with custom leather edgeroll and MBU-5 oxygen mask as pictured at Meiringen AFB in 1979.
Most probably, HGU-2s were provided to Swiss F-5 pilots during the aircraft evaluation or while attending the air combat training which took place at the USAF 425th Tactical Fighter Training Squadron, Williams AFB, Arizona.
Some publications also mention - during the period of the introduction of the F-5 in Switzerland - the "support" of US "Aggressor" pilots in dogfight instruction. Although there is no confirmation of this, in a 1982 television broadcast a Swiss Tiger pilot helmet (already a DH-151) sporting a USAF 527th "aggressor" squadron sticker is shown; a possible training at RAF Alconbury?
Given the previous Mirage III R&D and operational envelope boundaries widening in collaboration with the Israeli AF, a similar more or less advertised cooperation with the USAF cannot be ruled out.
In the Meiringen pictures there are also some anomalies that would confirm the "non-Swiss" origin of the helmets.
As a rule, "DYMO" embossed labels were not used by the Swiss AF to identify a piece of equipment as assigned to a particular pilot (see the name stripe on the back of the helmet and oxy mask shell identification); this was done by means of leather labels that showed only the pilot's allocated numerical code.
In addition, (see below LH picture) the number "80" visible on the oxygen mask shell at the time was not the personal code of the portrayed pilot but allocated to another one. In fact this labeling system was in place at the USAF Aircrew Flight Equipment Rooms; this would be the number of the locker/rack for storing personal effects allocated to each pilot.
More on Swiss AF HGU-2 HERE