The MP5K isn't the banned gun, it's the SP89 (which isn't an AOW). I've also heard a similar rumor that the days of "AOW != assault pistol" are now over. I am not really sure what to believe. Only way to know what the current opinion is is to call the MSP LD.
That said, you have one other option: use a reverse stretch pistol instead. These are not 100% parts compatible with the SP-89, and thus you should be safe doing an AOW with one. It'll be visually different, but that's about it.
