First - I made a live-usb of jessie-sysv using refracta2usb-0.9.6 in wheezy.
On booting into the usb, I get error messages about chain.c32 and vesamenu.c32, along with a boot prompt. (no menu). Typing "live" and hitting enter boots the system. I assume that the problem is the syslinux bootloader from wheezy doesn't match the syslinux files from jessie/sid.
Remove chain.c32 and the "boot hard disk" menu entry, replace vesamenu.c32 and isolinux.bin with the versions from wheezy, and it boots.
I then tried it again, running refracta2usb-0.9.6sid2 in jessie-sysv inside vbox. So the bootloader matched the other files. It may be significant that the syslinux command never finished and I lost mouse and keyboard control inside the VM, so I killed the VM.
When I booted the usb stick, I got the boot prompt with an error message about not finding isolinux/vesamenu.c32. Uh... this is a usb - the files are in /syslinux, not /isolinux. After some experimentation with copying or renaming these directories, it seems that both isolinux and syslinux folders are needed to get the boot menu. The two directories are identical now, with the exception of isolinux.cfg and syslinux.cfg, which are identical files except in name.
I'm gonna install jessie-sysv to hardware and try it again.