Has anyone considered decompiling eloquence? With something like ghidra or ida pro? Mario 64 was turned back into high level language source code this way.

This wouldn't be easy due to Eloquence's internal architecture. eci.[dll|so|dylib] only contains the low-level platform abstraction layer, things like threads, queues, mutexes etc, as well as utility classes for .ini file handling and such. It then loads a language module (from a path specified in eci.ini). The actual speech stack is statically linked separately into each language module (possibly with modifications, not sure about that); in theory, if you reverse-engineered the API between the main and language libraries, you could write an Eloquence wrapper for any arbitrary speech synthesizer. This means you'd have to reverse-engineer this separately for each language.

From what we know, Eloquence was compiled in two stages, stage1 compiled a proprietary language called Delta (for text-to-phoneme rules) to C++, which was then compiled to machine code. A lot of the existing code is likely autogenerated from a much more compact representation, probably via finite state transducers or some such.