Can't they be split into lines? OTR was designed for IRC that limited protocol lines (ie. payload line + command + extra fluff) to 512 bytes, so that ought to work on Discord too.
I have not yet tried, that may work since it does work for IRC (which also has a limit per message). It was just more of a proof of concept, tbh, but it works, just not as usable as it could be.
