Easy to see why it's not supported, but I however did get it working.
Open Disk Utility and create a New Image.
Set the options for the disk image in this order:
Image format: Sparse bundle disk image
Partitions: No partition map
Volume size: Custom (enter the size that you want it to be able to expand to)
I can't remember if setting the volume name matters or not, but if it doesn't work at the end of this you can come back and try it.
Save it to your local
computer with a name of your choice, it won't
take up the amount of space that you just allotted to it so don't worry.
Now copy it to the SMB drive you want to use. Rename the file to the filename that Time Machine wants to use, in this case Macintosh_001122334455.sparsebundle.
Start the Time Machine backup on the SMB drive and it *should* work fine, albeit slowly.