This is an interesting question and the devil will be in the details. A few basic things to consider:
1) Poly Tower's costs to operate will be partially influenced by the overall costs of the Polynesian Resort (of which, certain services should be shared between Poly Tower, PVB and Poly Hotel); this fact will remain regardless of whether Poly Tower is put into a trust with CFW, into it's own standalone association or into the existing PVB association (noting that in the case of being put into the existing PVB association, it isn't really distinct from PVB and then the cost sharing is just between PVB and Poly Hotel);
2) I have not seen anyone post copies of the actual purchase contracts/CCRs for CFW. However, from what has thus been described it sounds like when you purchase CFW you are purchasing points with a (priority/home resort) right to use whatever property is in the trust....and as such, I would expect the annual dues to (be split equally to) cover all properties in the trust (meaning if they were to dump Poly Tower or some future development into the Trust, you could see annual dues go down from the 12.xx they are now, if such development's overall annual costs were significantly lower). For example, let's say there are 100k CFW associated points in the trust with an actual annual cost of 12.50 per point to maintain. Then you add 100k PolyTower associated points (noting that the trust docs apparently require
DVC to deposit full "units" and not some fraction thereof) with an actual annual cost of $9.50per point to maintain. Theoretically the per trust point cost/annual dues would be a blend of the two (so in this case $11.00 per point);
3) It could be that regardless of the trust structure (and the ability to add another property to the trust) when you purchase, you are purchasing points with a (priority/home resort) right to use a specific property in the trust (and not any property in the trust), in which case it could also be that your annual dues are tied only to the costs for the specific home resort you have priority on. In this case, adding another property to the trust wouldn't impact the annual dues of "CFW points".
@Sandisw - curious if you've read or reviewed enough to know which of 2 or 3 is structurally permitted by the trust docs?