According to d_ed, the relevant function just hasn't been made invokable and exposed to the Javascript API.
A patch is being reviewed: https://phabricator.kde.org/D20044
Closed by https://phabricator.kde.org/R108:1d4a9d24f86799b45994c545b054e205f371869f Scripting interface now exposes Client.setMaximize(v, h) method.