>In my research and tests simply taking the change in the number of LOCs has been a surprisingly effective metric for sloppiness, with the ironic caveat that if we started optimizing for it, it would cease to be a meaningful measure.

This matches my experience. Before working on an issue, I ask the LLM to estimate net LOCs at the final PR based on the scope. It works well, and review steps do flag inconsistencies. But as the OP mentioned, if you turn this into a hard metric vs "design smell", you can see LLMs code-golfing for oneliners.