I have heard the same. If the intention is to marry then it's allowed for a woman to show her potential husband her hair. But this should all take place with presence of a third person and not alone.
Let me rephrase what I had meant earlier. As far as I heard in a lecture if there is a serious prospect for marriage the girl can reveal her hair if she wears hijab. There should be a mehram present and obviously this would not be done in public. This doesn't mean she has to/should sit there for hours. Just short period of time, enough so the guy who is seriously considering marrying her has seen her without hijab (headscarf) and has seen her hair etc.
Let me rephrase what I had meant earlier. As far as I heard in a lecture if there is a serious prospect for marriage the girl can reveal her hair if she wears hijab. There should be a mehram present and obviously this would not be done in public. This doesn't mean she has to/should sit there for hours. Just short period of time, enough so the guy who is seriously considering marrying her has seen her without hijab (headscarf) and has seen her hair etc.