You don't actually need to physically copy data, just create a view for every table that does a replacing merge between the original read-only data and the developer's own copy. And you can put a trigger on the view to redirect writes to the same private-copy table, making the whole thing transparent to the user.

Not disputing that Oracle might have had something like this built-in, but it sounds like something that I could have whipped up in a day or so as a custom solution. I actually proposed a similar system to create anonymized datasets for researchers when I worked at a national archive institute.

Sounds like a snapshot - a file based diff of the pages changed since the last full backup - easy to revert to for the same reasons.