This is about what I imagined those “mass credits” to be. But it’s actually a bit more complicated than something like a gold-backed money system. In a “matter based economy”, the matter is the asset you use to back your money, but at the same time you would want to optimize the physical distribution of matter for your own purposes. You’d probably end up with a sophisticated grid of “matter banks” that simply store matter for future use. Alternatively, you could try to maket the best use of matter in every second, but then it gets nasty when someone is trying to “cash out” on matter. You’d have to destroy some nice arrangement of matter (mostly habitats) in order to enable those who cash out on it to do something else with that matter. That wouldn’t fit well with those who have claims on those “nice arrangements of matter”. Having dedicated matter banks would remove that potential source of conflict.
This is probably easiest understood, if you think of matter or energy as being money. The value of that money will depend on the relation of the amount of money to the amount of “other valuable stuff” being close to a specific location. I’d guess that in the future the prime value will lie in location. Having lots of people and information around you will be seen as quite valuable, because cultural richness emerges from proximity of people and information interacting with each other. Even though there will be lots of matter in “crowded” places, the cultural value will probably still be much higher in comparison to sparely populated places. Matter would have a high value there, just as land has a much higher value in city centers than in the countryside.
When relocating matter from one location to another, one might also need to take relativistic effects into account, caused by large relative motions, and time dilation caused by large gravity fields – which is interesting, because it would slightly decease the value of large collections of matter at one place, compared to a more even distribution.
I don’t think it would be too complex. The problem I see is rather what to do in the case that people want to “cash out” more matter than is currently available in matter banks. Such possible failure modes do make it seem much preferable to stick to full (matter) reserve banking. Long term stability of the economy and its material basis would probably be much more important than sheer short-term efficiency, especially once people live indefinitely.