To make a large ram disk after boot, with no messing around with kernel parameters, this seems to work. Use tmpfs, make a file, mount it via loop, and mount that via a filesystem:
mount -t tmpfs -o size=200M tmpfs temp/
cd temp/
dd if=/dev/zero of=disk.img bs=1M count=199
losetup /dev/loop0 disk.img
mkfs.ext4 /dev/loop0
mount /dev/loop0 temp2/
Probably a bit of performance penalty going through multiple different layers... but at least it works.