Yes, that's possible. And it even works with built-in functions. Just edit your syslinux.cfg to include the following:
LABEL linux LINUX /boot/vmlinuz APPEND root=/dev/disk/by-label/data rootfstype=ntfs loop=images/linux_root.img loopfstype=ext4 rw INITRD /boot/initrd.img Obviously, you have to copy your kernel image (vmlinuz) and your initial RAM-disk image (initrd.img) to the FAT partition you want to boot off with SYSLINUX. (The path is relative to the partitions root.)
The root= parameter specifies the device and partition the root file system image lies on. You can of course also use the /dev/sda1 notation or the /dev/disk/by-UUID/SOMECRYPTYCNUMBER version.
The rootfstype specifies the file system of the partition the file system image lies on. NTFS and FAT are both supported out of the box. For more exotic formats you need to edit the initrd.img to include and load the appropriate kernel modules.
loop= gives the path to the root file system image relative to the base of the partition it lies on. loopfstype= specifies its internal type. It will, obviously, be mounted at / after boot. The partition the image was loaded from will me mounted at /host automatically if this folder exists. (You won't be able to mount it anywhere else if it is NOT mounted there atomatically. But you are able to move it to another mount point with mount -m if it is.)
Don't forget to copy and overwrite the kernel and the initrd.img on the FAT partition every time those get updated by a system update or strange things may happen on boot.
Unfortunately, there is no way of including a swap-partition in the same way. You have either to use a real partition for that or try to find a way to activate one after boot from the running system.
Expect approx. 10% CPU overhead for writing into the loop-device mounted through the NTFS-wrapper.