I noticed that, if the suffix of an .mrk file is renamed to any common archive format (.zip, .rar, .7z, .tar, .tar.gz, et cetera), it can then be extracted. The resulting directory presents the user with a 6-byte file called "header" and a directory called "rack" containing 35 files numbered 0-34, which I assume are audio files with some kind of encryption.
I don't understand why we're only able to use 8 samples when the factory sample format for Mellotron V involves what appears to be simple multisampling and would not be difficult to implement for end-users. If giving end-users the ability to compile their own .mrk is in the works, most excellent and more power to us all. But if not, it shows laziness on the part of Arturia. The time stretching option with which we are presented is novel and admittedly occasionally useful, but it is not on par with Arturia's tendency toward exacting replications.
If there's a good reason not to be able to load a full complement of 35 samples, I'd love to hear it.