"it is so demineralised that I recall reading somewhere that one shouldn't exclusively drink this water over extended periods of time."
Last month I was reading somewhere online, some fellow has published the recipes for ersatz Perrier, San Pelligrino, etc. for folks who want mineral water but don't like the price. A half dozen common and inexpensive chemicals (minerals) are all it takes, and you can "remineralize" your RO water and make it taste just like the $4-a-liter stuff.
How to make Mineral Water. Mineral water recipes.
Not the article I had read, but "make your own mineral water" turns up plenty of hits.