I've discussed the erroneous claim that Tatiana Konstantinovna's 1911 marriage was morganatic elsewhere, but as far as morganatic descendants being entitled to the surname of Romanov, this is clearly not the case.
In the summer of 1911, Nicholas II initiated a round of formal enquiries concerning the issue. The Emperor convened a special meeting of various Grand Dukes to discuss the forthcoming marriage and the question of allowing possible unequal unions, as there was much talk within the Imperial Family that Princess Tatiana’s marriage, if recognized, would set a precedent for morganatic marriages. In a letter written to the conclave by Baron Vladimir de Freedericksz on the Emperor’s behalf, the Minister of the Imperial Court declared Nicholas II’s decision: Grand Dukes would not be allowed to contract morganatic marriages, but Princes and Princesses of the Imperial Blood, on receiving the Emperor’s permission, would be allowed to do so. If they elected to enter into a morganatic union, the Emperor would grant new surnames and coats-of-arms to the spouses and to their descendants to signify the change in status. The Emperor further declared that henceforth there would be only two categories of marriages he allowed: those unions equal for the purposes of dynastic succession, and morganatic unions. A morganatic union was to be characterized by the granting of a new surname, as happened before the Revolution (i.e., Brassov), and after (Ilinsky, Romanovsky-Krassinsky, etc.)(Letter of Baron Vladimir de Freedericksz on behalf of Nicholas II, June 14, 1911, in GARF, F, 601, Op. 1, D. 2143) Morganatic descendants, therefore, were not, and are not, technically-by Nicholas II's decree-allowed to use the Romanov name. I use it with Nicholas Romanov and others because it's the courteous thing to do, but from a technical standpoint they are not entitled to it.
Greg King