You may just have to see when we build it.
Laying it down flat in your van in sections with blankets in between would be one way.
However, when it is modular as you would like, the frame will have to be rebuilt each time after breakdown.
The idea is to have Technic around the edges of the mosaic as you mentioned.
I have developed a way to build and transport that works for me after building a number of large mosaics 3'x4' and 4'x6' in size.
The use of plates to reinforce the build would not be sturdy enough due to the size of the mosaic and its weight.
The build also involves a large number of zip ties to make it sturdy along with a pipe system I have developed with Brian Costello.
These non-Lego (yikes) elements avoid gluing, drilling, screwing, and the use of wood (unless you prop the build up on an easel, which I have done in the past.
This build will take quite some time to assemble and construct (as everyone is used to).
Building outside of the parameters of my design based on the BOM is up to the group.
However, it will require many more parts and I have worked to try and avoid that with the builds I do since many parts are already required.