Yes its true that the AHARS will have problems and generate a heading mismatch but if your blocking someone then its not that hard to get around.
Not sure about the smaller ones but on the Q400 simply pushing "DG" at the holding point to the one thats going out of allignment then push it again when cleared to line up prevents the mismatch or for that matter just let them go out of alignment and once lined up a quick double push of the "DG" button will realign the the one that has gone 'out of wack.'
(I realise thats possibly applying more than the allowable amount of practicality to the situation though)