Just finished off a sack of MO and didn't have a problem with diacetyl at all. Stouts Porters and a few English ales and such and they came out fine but than I have been getting into the habit of using larger starters lately. 2 to 5 hours lag time depending on the starter. Rather than just throwing in a single pack or bottle of yeast I'll throw in at least a 1/4 cup or more to start out. If I can time the last ferment right I just dump in the new wort on the old cake.
Well, in many English style ales diacetyl is completely acceptable in moderate amounts especially depending on what strain you use. I always get a small amount of diacetyl when I use wlp002, for instance. Diacetyl isn't necessarily a bad thing. It can add mouthfeel and nuance to a beer. Some styles should not have any diacetyl, obviously, but in many English styles it can be very enjoyable in moderate doses. I personally think that MO highlights and compliments the moderate amount of diacetyl created from some strains.
And just to make sure everyone is clear, I am not saying I get diacetyl from using MO and I am not sure the OP was saying that either. I am saying I pick up some faint flavors from MO that remind
me of diacetyl. I should also say that I have gone through hundreds of sacks of MO and order it by the pallet now. I have been experimenting with 100% MO American IPAs and they turn out great, I absolutely love them. I am not saying in any way that it is a bad thing, though I have found it can be distracting in some recipes.
Also, I wouldn't really recommend going directly on a yeast cake unless you are going from a low gravity beer to a very high gravity beer. You are better off pitching the "right" amount of yeast rather that overpitching every batch.