Well, for DSA caps, they would all be interchangeable, which isn't true for DCS because of the different key profiles. For a DSA labelled GB even Colemak and Dvorak users would benefit from the same 1x keys as QWERTY folks. I could go for that, though I'd prefer DCS.
A poll might be a good idea, but my original thought was just to ask people to submit their layouts and retrieve the source code for each one. Writing a script to figure out what the popular keys are for the various positions wouldn't be too hard. For example, if 50 people use QWERTY in their layer 0, then we have 50 caps of the row1 profile that have 1/! on them, 50 caps of the row2 profile that have Q on them, 50 caps of the row3 profile that have A on them, etc. This could be a bit annoying to do from the source; if the web layout configurator could be made to spit out a simple list of positions and keys that would be much easier to parse than the default--layout.c file.
The 2x/1.5x keys, I bet, will show much greater variance, but it's certainly possible some keys would be very popular. I imagine most people are using the 2x keys for space, backspace, enter, and delete. The most popular options could be included in the GB, depending on cost, or else we could buy them ourselves, though I do rather like the idea of a full set being provided (perhaps along with a few blanks so that people can make some adjustments?).
At any rate, at its most complex, we could take a look at all of the layouts that people use. At its most simple, we could just list a bunch of layouts, set up a poll, and just go with whatever gets the most votes. In the middle, we could similarly put a vote on a layout and include some extra blank keys for the keys over which there is most contention/variance, and/or provide a couple extra copies in case there are two very popular options for what to do with certain keys.
No matter what though, it should be done at least a little intelligently in the sense of making sure that when keys are interchangeable, they're considered as such. Even if you and I put our +L1 keys in different places, if we both use the same cap type (depending on size/row) we would both get use out of having such a cap.