Is there any protocols to identify what sort of document MS-408 is?

[rikforto]

About entropies I'm not so sure. Lower-than-usual entropy is of course possible but I'd rather measure it on real conlangs before saying anything about it.

This is just a matter of creating a conlang where the letters order such that the uncertainty is lower than most naturalistic languages, which could be done. In Voynichese, entropy is almost certainly downstream of the qualitative lettering-ordering effects and whatever constraints created those. You could probably target a pretty accurate entropy value just by defining your word generation process to have exactly as much entropy as you wanted the final text to have and hit pretty close. Again, this would be bizarre, like Voynichese, but there really isn't a conceptual hurdle to restricting word formation to certain letter orders and, has been amply discussed, some real-world languages like Hawai'ian have alphabetic representations where we do see these constraints giving lower entropy values. It also would not capture some of the other oddities, like linestart words and apparent positional variation, but neither does the entropy summary statistic fully, so that's all good.