Automated mass round-trip testing on actual page content would be useful to ensure proper HTML round-tripping in VE. This is very similar to your existing DOM sanity check. Basically load it into DM, export it again and check that the result is identical.
You can probably reuse parts of our distributed test infrastructure for this (currently rt testing 160k pages from various wikis through Parsoid), and can directly use the cached HTML from production as the input.
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