> Have you ever tried "MAKEDEV sd47"?  Compare the device files
> made with those of sd15.  It does not do the right thing.

That's not true any more.

# ls -l sd15* sd47*
brw-r-----  1 root  operator  4, 120 Oct  1 22:20 sd15a
brw-r-----  1 root  operator  4, 121 Oct  1 22:20 sd15b
brw-r-----  1 root  operator  4, 122 Oct  1 22:20 sd15c
brw-r-----  1 root  operator  4, 123 Oct  1 22:20 sd15d
brw-r-----  1 root  operator  4, 124 Oct  1 22:20 sd15e
brw-r-----  1 root  operator  4, 125 Oct  1 22:20 sd15f
brw-r-----  1 root  operator  4, 126 Oct  1 22:20 sd15g
brw-r-----  1 root  operator  4, 127 Oct  1 22:20 sd15h
brw-r-----  1 root  operator  4, 376 Oct  1 22:20 sd47a
brw-r-----  1 root  operator  4, 377 Oct  1 22:20 sd47b
brw-r-----  1 root  operator  4, 378 Oct  1 22:20 sd47c
brw-r-----  1 root  operator  4, 379 Oct  1 22:20 sd47d
brw-r-----  1 root  operator  4, 380 Oct  1 22:20 sd47e
brw-r-----  1 root  operator  4, 381 Oct  1 22:20 sd47f
brw-r-----  1 root  operator  4, 382 Oct  1 22:20 sd47g
brw-r-----  1 root  operator  4, 383 Oct  1 22:20 sd47h
# 

> Even if "MAKEDEV sd47" worked, it is easier to refer to that 
> device as c2t15d0 than sd47, because the name makes more sense.

So rename it.  Or maybe even add support to MAKEDEV for creating the
{r,}dsk/cXtYdZ devices if you want.

> Could I then use /dev/c2t15d0 as a tape device?

No.  On the other hand, commercial systems put the different types of
devices in separate directories, and use different major numbers as
well.  So that doesn't further your point.

> If the system was
> booted with c2t15d0 turned off, and then I turned it on, could I
> then start dumping a filesystem to a tape there?

That's a hot-swapping issue, which is totally different.  There is in
fact a way to probe the SCSI bus for new devices, but it's not (yet)
automatic.

> Are there any mainstream UNIX (or UNIX-like) OSes which dynamically
> name SCSI devices on boot-up?

I'm not sure what your point is here.  If you're implying that we
shouldn't support this at all, then you've come to the wrong place.
If you'd like to have both mechanisms, that might be possible, but you
have to make a concrete proposal (which you haven't done).