Baron von S, IMO (and that of some but not all others), the OA rule is simply a clarification of the PH rule, which is (IMO and that of virtually everybody else) very poorly written and unclear. In any case, the OA rule is exactly what I was using based on the PH text, even before OA came out ...
Corwin, an interesting HR I've seen suggested to avoid shafting the multiclassed monk is to change the improved unarmed attack progression to a special ability tied to the monk class levels. Eg. a character with 6 monk levels has the special ability that, when unarmored and using unarmed attacks or 'special monk weapons', he gets a second iterative attack at -3 instead of -5. Any additional attacks occur at the standard -5 progression. So, a Ftr8/Mnk6 (with a normal attack progression of +12/+7/+2) would have an unarmed attack progression of +12/+9/+4. (Vs. +12/+9/+6/+3 in SKR's system, or +12/+7/+2 under the standard rules.)
I haven't playtested that, but it looks pretty good to me ...