I read somewhere, someone asking this question, maybe it was on the mail-list. The reply was, yes, it can be done.
Is that still the case? If If so, for the default set to install into /, what variable would I have to change?
Perhaps some recipes are hard-coded to install into /opt? -- when I first started writing recipes, I am guilty of that. But then, that should be ok, some packages can be installed into /opt, even though the default is /.

Just an extra note: I wrote some code that converts all of the symlinks into /opt, to hard-links. And symlinks-to-symlinks changed to symlink to the local hard-link -- for example /bin/ls was a symlink to /opt/busybox/bin/ls, now becomes symlink to /bin/busybox.
The result is faster operation, smaller size (as hard-links are smaller than symlinks), and looking at the file with a graphical file manager, it looks like a normal filesystem, instead of a window full of symlinks.

In fact, after doing that, most of /opt can be deleted, if desired for any reason. Although there is /opt/busybox/bin/busybox and /bin/busybox, they are hard-links to the same inode, the same file, and deleting one of them does not delete the file. Both hard-links have to be deleted.

...this is actually a very roundabout way of ending up with a "normal" filesystem, with most pkgs in /.