Dear M. L….
Darn tootin’ you shouldn’t wait forever. This agent knows that, too, as she acknowledges with her request for notification should you land other representation. Send her a note that you’re starting to submit a new project elsewhere and should it garner an offer you’ll let her know the ms she has is off the table. You’re neither pulling the dystopian submission from her nor submitting that same ms to others, so there’s no hint of foul here. In fact, you’re taking an extra step to be communicative and she’ll likely note that professionalism. Her concern isn’t personal, she just doesn’t want to spend time reading an ms that’s not available. End your note by saying you know agents like to consider only one project at a time from an author, but if she’d like to see the new one you can send it her way, too, as part of your multiple submission. “It’s a [insert genre] about…” and then supply your one-sentence hook statement about the story. If she’s intrigued, she’ll ask for the new submission and then you can provide your full query pitch for your fab new project.