In traditional grammar, synesis refers to inflection being determined by underlying semantics instead of morphological agreement; the most familiar instance in English is expressions like The government are divided.
In the 1935 reference grammar of Esperanto, the term sinetio is used. This is presumably the same word, possibly because the authors did not like the sound (or recognisability) of *sinezo. (The Dutch and English entries in Majstro.com think so. On the other hand, the Spanish entry guesses it is derived from synaetia rather than synesis > *synetia, and calques it as concausa.)
The authors (the French scholar Gaston Waringhien and the Hungarian scholar Kalman Kalocsay) use sinetio in a sense related to synesis, but much broader. They use it to refer to what traditional grammar discusses under syntax, but we nowadays would shuffle between morphology and semantics: it's the semantics and distribution of functional morphemes and words. So, what does conjunction X mean, when is the accusative used, how does the use of the past tense in subordination correlate with the tense of the main clause, etc. The kind of thing that traditional grammars of the free-word order Classical languages spend almost all their time discussing under "syntax".
1935 is quite early for any post-traditionalist discussion of syntax, particularly among people who weren't professional linguists (Waringhien was a schoolteacher in his day job, Kalocsay a doctor), and so probably did not know what Bloomfield was up to at around the same time. So I'm assuming that, even if they innovated in the morphology of sinetio, they didn't innovate in its usage: that there was some precedent in traditional grammar in differentiating Classicist notions of syntax ("the ethical dative, the subjunctive of intent") from syntax proper.
Has anyone found a similar extension of the usage of synesis outside Esperanto? Or an attempt to differentiate semantics of functional morphemes from syntax before the structuralists?