Yes, it is possible by installing the Wyoming Satellite software on the Mark II. All the pieces are there, it's just missing some easy-to-install firmware.
Source: I worked at Mycroft on the Mark II and am now the voice guy for Nabu Casa (Home Assistant).
I'm the author of Piper; it is a successor to Larynx (originally named Larynx 2). Piper uses the same underlying model as Mimic 3, which I developed before joining Mycroft. However, Piper uses a different library to get word pronunciations, so the voices aren't compatible between the two projects.
It's been an awesome year so far with Nabu Casa, and I'm very fortunate to be able to work on something I love. I hope to contribute to the open source voice space for many years to come :)
Coauthor of the blog post here. You're right, I said it on the live stream we had today but forgot to mention it in the blog post: the i5 is from a Lenovo ThinkCentre M72e.
They're available refurbished for less than the cost of a Pi 4 these days, so it seemed to be a good comparison!
Home Assistant and Rhasspy 3 will be able to share voice services thanks to the shared Wyoming protocol.
Rhasspy 3 will have more options, including lots of experimental services.