I read your documentation, and, as far as I properly understand it, the lexical constraints can only be provided at the ONMT server starting time. Right ? Is it possible to send a list of lexical constraints with a sentence to be translated ?
I think it would be a nice indirect way to solve this request:
Until that happens I have introduced a Custom Dictionary feature in our MT Gateway (client) so that the user can force a particular translation. The Network seems to handle these translations acceptably in the case of nouns & noun phrases, which are usually what need to be “pre-translated” in our particular use cases.