Right, RAM is part of the SoC... that probably makes the idea pretty much a no-go.
The RAM was only integrated on the SoC on earlier models (called a package-on-package, PoP -- see https://elinux.org/RPi_Hardware#Components)
Later versions (I think starting with RPi 2) have a separate DRAM chip on the bottom side of the PCB.
Not to say Broadcom couldn't spin a newer revision of the SoC with a different internal architecture, but for now it seems that >1 GB of RAM would require substantially more effort than just swapping the DRAM module on the board.