Thanks for providing such a good, clear drawing. As the saying goes, it is worth a thousand words.
As a first suggestion I'd say you should run this by the designers at Lowes, Home Depot or other cabinet supplier. They run into these situations all the time and most likely have several solutions. That said...
Am I correct in reading that you want a door on the outside of the peninsula, at the uppermost arrow, near the 3.75? if so, that would definately require some custom work, as base cabinets typically have a drawer at the top and I have to assume you wouldn't want a drawer there. To be honest, I'm not sure you would even want a door there. It just wouldn't look right to have the blank panel on that side of the peninsula with a door all the way at the end. A better soution would be to use a 21" cabinet at the outer end of the peninsula and what's called a "half blind" cabinet at the end toward the sink, with the same size door as the 21. The half blind cabinet would be a custom size.
See the standard dimentions here: http://www.kitchencabinetmart.com/kitchen_cabinet_sizes.htm