It may be true that the 280 wheels don't have the nubs needed to hold the 230-style hub caps (as opposed to the full wheel covers) in place. However, the 230 wheels will definitely also hold the 280-style wheel covers, as shown by the number of 230s with the incorrect covers on them.
When I bought my '66 230SL, it had the 280-style covers on the wheels, and I put some effort in converting back to the hub caps and beauty rings that belonged on it (much nicer, IMHO).
In Andrew's case, he has the 230 covers on his 280, so it may not be true that the 280 wheels won't accept them.
Andrew, I still have my 280-style full wheel covers, and if it made sense from a shipping cost point of view, I would be happy to swap with you. I'd have to get them from storage and check out their condition, but I believe that they were in nice shape.
-David