This patch implements a UV Select Overlap operator. It was first attempted by another volunteer as part of another change (D2865) but it was never updated with the feedback given.
The patch below incorporates said feedback as well as allows the operator to work correctly in multi-object edit mode.
The 2d tri-tri overlap test exhibits good behavior in a variety of cases (e.g. very tiny overlaps in a crowded UV layout; no false positives) but my corpus of available UV's is limited. It detected a valid overlap with the 2.80 splash screen .blend and it handles my test file which will be provided below.
Performance: A 100k vertex model takes about 0.5 seconds to detect overlaps (full operator cost) on a Xeon E5-1620 (6 years old). For a rough comparison against how long UV operators sometime take, a UV Smart Unwarp for the same model takes 14 seconds.