Can a Muslim ask a hujjah to declare takfir on one who denies Quranic concepts such as hijab?
It depends on what exactly the person says and means by his words.
If he knowingly denies a verse of the Quran or invents an interpretation that goes against what is apparent and held by consensus then he is a disbeliever, because among the things which nullify a person's Islam are:
أو حلل محرما بالإجماع كالزنا أو عكسه أو نفى وجوب مجمع عليه أو عكسه
That one considers lawful what is forbidden by ijmaa, e.g. fornication ; or that one denies what is obligatory according to ijmaa.
Specifically regarding Hijab:
But if what is meant by hijab is covering the head, then there is no difference of opinion among the scholars concerning the fact that it is obligatory. His rejection of both types, and his making fun of them, is apostasy from Islam ... so denying it and making fun of it is kufr which puts one beyond the pale of Islam.