This is a pretty common request, the most compelling argument being the idea that if we implemented the architecture people around the world could make and share plugins expanding Delicious Library's functionality and lowering our workload.
I have my doubts, and I'll tell you why.
Delicious Library's data is wide open in XML, a format understood by even the most novice of programmers. Even without a plugin architecture, there's nothing to stop anyone with a decent knowledge of any programming or scripting language from watching the data file, picking out failed lookups, looking up the information, and inserting the results into Delicious Library.
The fact that, in two years, nobody's done that leads me to suspect that nobody will write plugins either.
Of course, that's not to say we wouldn't ever write a plugin architecture, even if for our own use. It's just good design. However, I don't think plugins are the panacea they're made out to be.