You write either a MG or YA book, and hope to catch an agent's attention. You *can* say that the book is a standalone with series potential. You *might* say it's got crossover potential. That's it.
I won't say anything unless prompted by Agents, Editors, or Publishers. The first book is what it is. It will be YA-/MG+ (agewise), but I will pitch it as YA. The MG elements will, if I write them correctly, will be seen as quirky. I have to be judicious with these, as I do not want to turn off YA readers. Like I noted, this is an experiment to see if I can cater to interests of both, concurrently, without adding anything to mature or juvenile to lose interest of either MG or YA readership. This is my first book. This is my experiment and my ability to sell my skill at writing.
I understand what you mean, but I think everything is not as black and white as one or the other. This is subjective, especially for readers. After all, many adults read Rowling's books, and early books were geared for MG readers.
Cheers.