Minch,
Since no one here actually answered your question I will chime in and give you the answer that you desire. This is a very simple solution. What you do is use your condition menu(s).

Copy the default condition into one of the free conditios for that model.

Go into the trim settings and set your elevator trim to be separate and not combined. This is annotated next to each trim control in the menu by either a C for combined or a S for separate.

Then set your second condition to be on when you flip the switch to put up your airbrake. By doing all of this you don't need to do any fancy mixing with the pmixes and by setting your elevator trim to be separate from one condition to another you can have separate trim settings in each condition.

I know it sounds kind of difficult but if you need help just email me and I will be more than happy to walk you through it on the phone.

patrick.