First, what's worked: I've had no trouble running multi-boot systems
with FreeBSD, various flavors of Linux, DOS, and Windows on hard disk.
I've also gotten (to my surprise at first, because it was so simple)
FreeBSD running on a thumb drive, including X and a slew of apps.
Every now and then I have a driver issue but otherwise it's proven to
be a very versatile and reliable tool.

What is hanging me up is that I cannot get FreeDOS to boot from a USB
stick slice, although it boots fine if (for example) I image the
entire thumb drive with a DOS disk image. Here's what *isn't* the
problem:

1. DOS is always on the first partition, so it's not an issue with
trying to install it on a different one.
2. I've tried this with multiple disk images and had identical
results.
3. I've tried with multiple boot loaders (Grub, LiLo, FreeBSD's) and
had the same results.

I guess what I'm looking for is a better understanding of how a -nix
bootloader runs DOS, and what might be different about booting DOS
from a slice as opposed to a whole thumb drive (which obviously isn't
an issue on hard disk). Usually when I set up a multi-boot system on
hard disk, I install the DOS or Windows partition first, then the
others plus a -nix boot loader, and have had zero issues. Also, if
someone can clarify the proper way to image a bootable DOS disk with
Unix or Linux this would be appreciated. Right now I'm using FreeBSD's
sysinstall, installing a file system and boot blocks with newfs_msdos,
then copying the bare minimum of necessary DOS files. I've also used
dd with prebuilt images. If I find a solution I'll post a follow-up,
but in the meantime suggestions are appreciated.