I am re-researching this topic. I am sorry but I don't have a definitive answer at the moment.
As you are probably aware, the external dimensions of the P226 are larger than the P228 & 229. The dustcover and grip are longer, the takedown hole is in a different location, and while many of the internal parts are compatible, the barrel and slide of a P226 will not fit on a stock P228 without modification of one or both.
Previous KTO Sig paperweights (in the 2000-2004-ish era) were designed w/ only the P228 platform in mind. Some folks managed to use P226 parts on them and do "hybrid" builds (one guy even called his a "P227"
). But in general doing so meant a lot more design/fitting/machining work.
When the idea for this GB came up last April or so, there was talk of making the new version of the paperweight so that it could be compatible with P226/228/229 parts. Richard felt he had come up with a design that would allow this (as you described), and that was what we have been assuming all along. But your concerns are valid, and none of us have actually seen the new design, let alone an actual paperweight. We do know he has gotten BATFE declaration for the sample he submitted. If he has another on hand, we can ask for pictures and such. If not, we may have only drawings.
The long time involved in building interest for this GB resulted in a bit of a rush to proceed on my part, once it appeared we had reached the minimum order needed. The discussions of the design and build particulars were spread out over several threads and about a year. But it is important that we get this resolved before any $$ changes hands. Everyone needs to know exactly what they are buying, what they can expect to build from it, what to expect in terms of the operations needed for their desired build, and what parts will be compatible. I will work on collecting that information, and if confirmation and payment deadlines need to be extended, we can do so.
As an aside, "opt outs" are keeping pace with confirmations at this point. Most folks opting out mention financial reasons, rather than not wanting to participate, if that matters to anyone. In any event I'll update totals soon, and hopefully we will still be well above our minimum order when everyone has responded, but I'm not at all certain that will be the case.
Stand by, folks.