About your edit, I highly doubt that a professional linguist would describe something "incorrect" or "wrong." Linguistics does not judge correctness or define what is right. It's totally against the concept of descriptive grammar.
The professor you're talking about has absolutely zero authority in defining what others should pronounce OO and XX, or any word for that matter. Even dictionaries never dictate the actual usage, meaning, and pronunciation of a word; they only try to describe the current state of the language.
To answer your question, these placeholds don't have "correct" readings. If you're having a hard time swallowing this, ask yourself what the "correct" readings of underlines and blank boxes in a fill-in-a-blank test in English. A cloze test may have a question like
He __ John.
where you have multiple choices: 1. is, 2., are, 3. am, 4. be. and so on. So, what's the "reading" of this underline? The placeholder in the next question could be a blank box. Would your professor claim that underlines and blank boxes must be pronounced differently because they must be distinct lexical items?
Placeholders are placeholders. Nothing special about them. "○○" and the like are pronounced まるまる (most likely reading for this particular placeholder), なになに (most generic of the bunch), だれだれ (when referring to a person), ほにゃらら (a bit humorous), and many other ways. Your first instinct "マルマルするにはバツバツすぎる" sounds perfectly natural to me, too. In fact, I think that's the most likely reading native speakers would use.
If you want a real example that backs up your instinct, here's an official document (application form for tender on construction contracts):
On page 13 (or page 15 in PDF count), there is a sample instructing how to fill out the form. There, you can find a bunch of placeholders including ○○, ××, and △△. And some of them have readings on top of them. For example, 日本○○設計 has kana "にほんまるまるせっけい" there. 代表者の氏名 (name of representative) is filled as "○○ ××," which has まるまる ばつばつ as their readings.
You can also read the Wikipdeia article on 伏字 here:
伏字 is a special use of placeholders such as censoring certain letters. It's a bit different than your example. But you can learn how ○ and × can be read in this case there.
Anyway, if someone reads a placeholder aloud in a specific way, if everyone understands what he means, and if it sounds as good as any other, that's perfectly fine. Your professor can insist that it's "incorrect" to pronounce underlines and blank boxes the same way as much as he or she wants. But that's not gonna fly in linguistics. His or her doctrine has nothing to do with how we, as Japanese speakers, native or nonnative, should speak Japanese. It's none of that person's business.