Probably not helpful, but "Mostly Harmless" is also the title of Rob Rhinehart's personal blog[1]. Rhinehart's 2013 posts about his nutrition experiments under the "Mostly Harmless" title, led to the founding of Soylent.com and the modern meal replacement industry.

So just possibly MH was a fan of Soylent, or of DIY Soylent.

[1] https://www.robrhinehart.com/

Edit: on this notebook page[2] are the words Algae Pool and Nutrigs (nutrients?)

The pages[3]-[4] seems devoted to planning or documenting an energy-bar recipe. Similar fascination with the nutritional details of artificial foods are expressed by hobbyists who dabble in DIY Soylent recipes, see for example [5], [6].

On this basis I think it at least credible that MH was aware of Soylent and DIY attempts at complete-nutrition foods, so might have learned the "Mostly Harmless" title from Rhinehart's blog.

Which is no help whatever in identifying him, but interesting.

[2] https://www.dropbox.com/sh/hkpqsnk858pzu7f/AADnCzr7tabArQ_xq...

[3] https://www.dropbox.com/sh/hkpqsnk858pzu7f/AADnCzr7tabArQ_xq...

[4] https://www.dropbox.com/sh/hkpqsnk858pzu7f/AADnCzr7tabArQ_xq...

[5] https://www.reddit.com/r/soylent

[6] https://www.completefoods.co/