I'm now back from my long haul trip and starting to download the data to Universal Cartographics. I spread the downloads over a number of stations as I believe it helps with local rep building, so dumping ~20 systems worth of data per system.
First Discovery seems to be working in real time. So if I have a system's data displaying for 10,000 credits, when I sell it my balance might go up by 15,000 and then I get a pop-up window as part of the selling process (a bit like a teleprompter, one character per time), that I've made a further 5,000 bonus credits for being the first discoverer. BUT, the value of those further bonus credits is added to my balance when first selling the data, not when the bonus screen pop-ups.
Finally, flipping back to the Galaxy Map and then the System Map for the place I've made first discovery my commander name appears straight away, including for 'mixed' systems when I've only discovered some bodies.
All in all I'm very pleased. I'm selectively selling down one system's data at a time, rather than selling a page of 20 in one go. I'd imagine the "first discoverer" popup would then list all bodies sold on that page of 20 systems.