Wrote to a contact at Gentex (helmets) who pointed me to a group of retired life support/flight equipment guys. I started received replies to my enquiry (re: HGU-7's and who was issued them). None reference the Bird Dog but give us a starting to point to see who was issued them
"I was in B-47 and KC-97 at Lincoln AFB Nebraska from 1959 to 1966. We used them."
"HGU-7/P helmets were issued to C-130 crews in the early 70s but were transitioned out in the 72-75 time frame. Normally C-130 crews only used headsets but when they had to go on Oxygen they would remove the headset and don the helmet that also had a headset and bayonet receivers for the oxygen mask.
"The helmet were also issued to med evac crews (nurses, med tech etc...) that would fly on C-130s when the airplane was configured for med evac missions. The use was the same as the aircrew. Med evac folks would only don the helmet when there was a need to be on oxygen."
"I was stationed @ Pope AFB from Jan 64 - jun 67 in what we called "Personal Equipment" later Life Support with C-130Es. Our crews used this helmet which was designed to be a "minimum protection system with communications." Most preferred to take the visor off since it tended to ride forward and pull the front of the helmet down. The cushion pads didn't stay attached very well either. As I remember no one liked it very much. In fact we had several crew members who continued to use their MB-3 leather helmets."