[solved] rw-snapshot from ro-device?

[_Razorblade_]

I'd like to create a writable snapshot (with cow data on a ramdisk) from a read-only device (accessed as iscsi device).
Classical LVM snapshots are of no help here as the snapshot volume needs to be in the VG of the original data, but I only have read-only access to the volume, so I cannot change the metadata (add the ramdisk to the vg). Even I could, I wouldnt want to do this, as the original volume should be completely unaware that there is a snapshot.

It might be possible using dm-setup to manually create a snapshot, but in the very few examples that I have found this includes suspending the origin volume and moving the tables around which is not possible with ro-access either.

I'm not so worried about writes/data-changes to the origin volume while the snapshot is alive (which is is the primary reason for remapping access to the origin volume to a snapshot-aware volume, right?) and I also do not care about the data that changed during runtime on the snapshot (if I was, I wouldnt use a ramdisk, haha).

Any ideas how to do this?

I also thought about using a union fs, but I dont want heavy kernel-patching and out-of-tree modules...

Edit: I found a few good examples here:
http://linuxgazette.net/114/kapil.html