Does it have to be the imperfect of the subjunctive?
Not mandatory, but still sounding quite well, at least when my eyes are hearing it...
It’s easy enough to avoid it:
- Mais peut-être se jouait-elle déjà de moi, à l’époque...
Further down is a quick, totally unscientific, but nevertheless real-life test that suggest this might well be your best bet if you want to mimic the real-life day-to-day contemporary oral use of French.
What if we want to preserve the subjunctive?
Keeping the subjunctive is a bit harder. The present tense is totally out of question because of the introduction with “à moins que”, which demands the past in this context. Since the imperfect of the subjunctive is so scarce in today’s speech habits of French speakers, and since there’s a clear need for a past tense here, people would tend to re-invent a conjugation of the imperfect of the subjunctive in order to maintain the past rather than switch to the easier present tense of the subjunctive, like it’s commonly being done in other contexts.
The structure remains that of the subjunctive, since it occurs in a context where the subjunctive would always show up, were the sentence to be in the present tense1. But it tends to mimic the conjugation of the indicative for some persons, including the third singular person in this case.
→ N.B. Obviously, this is unlikely to be recommended by any serious grammatician, but it will “sound” better than the present of the subjunctive:
- À moins qu’elle (ne) se jouait déjà de moi, à l’époque...
To illustrate an example where the conjugation may differ from the indicative, let’s switch to the second plural person. In this case, I, Montée de lait, would tend to say something like the following (99% of the time without the “ne”, but this is another subject altogether):
- À moins que vous (ne) vous jouiiez déjà de moi, à l’époque.
This conjugation is not to be found in textbooks (they’d rather recommend que vous ne vous jouassiez in this case, but this conjugation has grown pretty much foreign to today’s spoken French). However, the presence of an added “i” and the extended pronunciation it triggers makes it feels like a past tense, and differenciates it from the subjunctive present (que vous ne jouiez).
How well would that new type of conjugation pass with real people?
I tested around me with boire and the second plural person, and everybody in my sample said they would rephrase for lack of knowing how to conjugate the verb at that point. Their answers kept “à moins que”, but were otherwise rather longer and using a conjugation of the indicative mode:
- À moins que vous aviez déjà commencé à boire à l’époque.
When asked to comment on my answer of it, “À moins que vous buviiez déjà à l’époque”, they all said they wouldn’t have come up with it, most said they probably wouldn’t use it in the future, but that it did have a clear past tense feeling to it and was quite easily understandable.
So use with caution...
- Je modifie le verbe utilisé pour bien montrer le subjonctif, la conjugaison du verbe jouer étant la même au présent de l’indicatif et du subjonctif pour la troisième personne du singulier :
- À moins qu’elle ne se dise déjà de moi que je suis bien naïf.
Extras
Je ne voulais pas trop m’attarder sur le sujet, parce que la question est légitime. Elle concerne une tournure appelant normalement un subjonctif imparfait et le contexte est accessoire, il a pour but d’illustrer d’où surgit la question.
Mais on a en commentaires mentionné quelques faiblesses de l’exemple. Elles sont possiblement réelles et je leur donne ce petit espace:
- Enfant, je jouais souvent avec elle. → Ce n’est pas elle mais je qui était alors enfant, ce qui n’est possiblement pas le but.
- The OP was actually caught a bit by surprise with this one (see below in the comments). There are ways around, but they may not work in the bigger picture (which we don’t have):
Enfant, elle venait souvent jouer avec moi.
Quand elle n’était qu’une petite fille, je jouais souvent avec elle.
- Se jouer de quelqu’un est assez rarement positif, et assume tout un moins un masque : au mieux, elle ne prenait qu’un intérêt de façade au jeu ou faignait d’être plus naïve qu’elle ne l’était pour endormir la méfiance et remporter les enjeux importants.
- This point is clarified in comments by the OP. There is indeed a will to emphasize her manipulative mind.