can you use the extensions with the crown moulding wing?
No! The KA-UG is mounted directly on the Kapex (into the V-Kerf), but as you can see there's the track on top and two under the extension and it should be easy to attach a clamp or some DIY jig. I guess Festool will come up with a solution some time, at least the two tracks would make me think so.
very interesting - if you had two MFTs connected, could you connect two of these to one side to support a full sheet of plywood?
If you connect one left and right to the side of the MFT, the track/ rail would guide your sheet perfectly (the attackment it really level and rigid), since it can be mounted at the end or max cutting width of the MFT, or for that matter at the end of the MFT extrusion. On the other hand the track is a little higher than the rest of the extension and therefore not flush with the MFT surface, but since your cutting width is limited to the MFT anyway I would say the two KA-UG would perfectly suit this purpose if you have the two extensions for the Kapex anyway.
I have already looked at the spare part list of the KA-UG and if you're looking for an extension for cutting sheets, you could order two of the KA-UG V-attachment plates and attach your own extension with or without the Festool legs (although some leg support is needed). The plate will probably cost 15 US$ times two. I am also still working on a solution for attaching a router table (cheaper than buying the MFT-VL and CMS Module), but it will probably take some time. I keep you posted.