I agree with the other answers that 男前な女 refers to a woman with a somewhat masculine quality to her looks, in a positive way.

However, I think that "tomboy" is the wrong English equivalent. It does not match with your description of the context where the woman in question is "repeatedly described as a conventional beauty."

A tomboy is a woman who acts like a man or boy (depending on age) , and may dress the part, but it is almost entirely her behaviour. Whether she is attractive, whether in spite of or because of her tomboyish, is incidental.

On the other hand, it is perfectly acceptable to describe a woman as handsome. It refers to her physical features and implies that she has a sort of masculine quality which adds to her feminine charms.

Thus, I think the best translation for 男前な女 is "a handsome woman".

I agree with the other answers that 男前な女 refers to a woman with a somewhat masculine quality to her looks, in a positive way.

However, I think that "tomboy" is the wrong English equivalent. It does not match with your description of the context where the woman in question is "repeatedly described as a conventional beauty."

A tomboy is a woman who acts like a man or boy (depending on age) , and may dress the part, but it is almost entirely her behaviour. Whether she is attractive, whether in spite of or because of her tomboyish, is incidental.

On the other hand, it is perfectly acceptable to describe a woman as handsome. It refers to her physical features and implies that she has a sort of masculine quality which adds to her feminine charms.

Thus, I think the best translation for 男前な女 is "a handsome woman".