nosyparker: It is Hagaon HaRav Dovid that said what you reference, that the teshuva was a personal heter for an exceptional case. Rav Dovid Halpern shlita wrote about the question to Rav Dovid on this and that was Rav Dovid’s response. HaRav Matisyahu Soloman shlita also wrote a letter saying he met Rav Dovid Feinstein and Rav Dovid agreed to publicize Rav Halpern’s above letter. Rav Avraham Pam also stated that it is clear that Rav Moshe did not give a collective heter to leave some hair uncovered. Rav Nissim Karelitz also writes that Rav Moshe was only allowing it in a specific special case.
The Chasam Sofer and the Chazon Ish have issued a psak forbidding any hair from showing.