Platform sprite collections actually do work with non-multiple of 2 scales, just add what you want in tk2dSystem.assetPlatforms
I would set up something between 1x and 2x, to get the scale correct for your new collection.
The idea was to allow users to set this up in the editor, but it was too error prone - it was too easy to set things up wrong.
The other thing with platform collections is that you can tweak the resources directory, so you don't include all platforms in the build. Again this isn't offically supported, but its straightfoward enough - rebuild the index, then go throug the resources folder finding the tk2d_ files in there, loading them in and then deleting the ones with references to any platform you don't want. The whole system is designed with this in mind, but as I've never required this in our own projects, this was never implemented.
Hope that answers your quesiton...