I still don't see it. Let's simplify the hypothetical: A taxpayer is an HCE in two unrelated companies. He defers exactly $12K in each plan. After each plan completes their respective ADP test, this HCE's $6,000 in excess employee contributions (in each plan) are reclassified as catchup.
I think: For each plan, the HCE remains within that plan's limit. Overall, the participant remains at the individual 402(g) limit for a single taxpayer.
You're saying that because catchup is limited to $6,000, his legal catchup limit is $6,000; and completely ignores that fact that he has a 402(g) limit of $24,000 that isn't exceeded. I don't agree with that. I don't believe a plan of one employer can dictate to another plan what is a catchup. When it comes to the statutory limit ($18,000 plus $6,000), this is the only time the plans (of two totally unrelated employers) should be combined for this determination.
Good Luck!