I think the Preset use customized Bricks, but that the Brick is applied to the entire Image (iow, there is no masking involved). Different channels recieve different treatment -- but again, there is no mask.
Any Preset can be exported and/or imported.
If it turns out the Presets do use custom masks, please report back. Thanks.
You may find that this is the answer (or not):
Note - personally, I just work in Photoshop if the process is going to be that involved.
The article that Corky02 referenced is very vague about what the author is doing - the author does not like to give away his secrets, but nevertheless wants to show off his skills.
The user library contains greylevel mask tiffs that are describing the shapes of the brushes, located at
/Users/your short user name/Library/Application Support/Aperture/Adjustment Preset Brushing/
with subfolders named eg. p/p8/, containing the tiffs, eg. p8GC3nK5SeG216GSo9ewOQ.tiff
It is very difficult to guess which mask relates to which preset, the only clue to that is hidden in the property list
/Users/your short user name/Library/Application Support/Aperture/Adjustment Chain Presets.plist
You could try to browse this file with Xcode and look for the preset names, if you have the Developer Tools installed, maybe it is described there, but that would be like seeking a needle in a hay stack.
And even if you locate the correct preset name, it woud be walking a tightrope between success and failure - I would not risk it.
Rereading the above article I rather think the author was suggesting to edit the masks in the Aperture Library, not the preset masks in the User Library.
If I apply a brush to one of the images I see the path of the brush stored in a mask in the "Masks" folder of the Aperture Library - nearly impossible to track down, if the Library is large, only the date is a clue, but here is an example of such a mask, after using the sharpen brush on the picture of an orchid: I modified that mask with Photoshop CS4 to close gaps in the contour.
I do not find this method very useful, it takes a long time to identify the file to be edited, and it is easy to accidentally corrupt the Aperture library while doing this.