Hi Percy,
We are actually in the process of reevaluating our SSML compatibility. As it stands right now, Cepstral voices are not compatible with the lexicon SSML tag, so specifying a PLS dictionary would not be possible. The only option would be to edit your lexicon.txt or to pass a lexicon to swift with the -l parameter. In both cases, the lexicon would need to be formatted according to Cepstral's lexicon specifications (
http://www.cepstral.com/en/tutorials/view/lexicon). Our SSML compatibility will likely expand in the near future, so we will keep you posted if there are any new developments.
Nichole,
The Cepstral Support Team