Linux kernels 2.2.17 and above are supposed to have the Onstream support built in, and there are kernel patches for 2.2.14 and 2.2.16. You can patch your e-smith kernel with the 2.2.16 patch (of course you'd have to install the kernel sources and compiler) and then use the taper module for your backups.
The 2.4.x kernels also have the Onstream support in the kernel that best work with the ide-scsi interface and the /dev/osst and /dev/nosst devices. I have an Onstream drive that I'm using contentedly with SuSE 7.1 (2.4.2 kernel) and Taper. However, since I have a SCSI drive for my e-smith box, I see no compelling reason to plop the OnStream drive in it.
Honestly, getting an OnStream device working on e-smith probably would not be a big deal if you know how to compile a kernel and apply patches. I don't know if the flexbackup will work with it, but there's either a how-to or rpm for Taper that you can use from the backup module of the e-smith module (check
www.esmith.org).
Don