I'm going to cheat, and answer my own question:

Yes, subject to these conditions and considerations:

1. The new side nav group must be a standard main page, and needs at least some content. For me, it was an already existing page that is basically just links to the other pages in the new nav grouping, with a little content to explain what the group was about.
2. You have to "tweak" or "hack" the Anchor Text to make its appearance and positioning like other side nav group headings.
3. Since the new side nav group heading is in fact a main page, "Manage Pages" and moving pages around between nav groups is basically the same as before. All pages under the new side nav group are still main pages, not sub-pages.
4. Since everything is standard html, this technique should not affect anchor text SEO. But even if it does, for my purposes, I don't think it matters for this particular page.

How it's Done:

In CMS2, the Anchor Text box for a page supports at least some basic html code. I haven't found the limit on number of characters yet, but I am sure there is one. Playing around a bit on my template site, which uses a pretty much standard REW 1024 template with left side navigation, I created a new main page for the new side nav category, and gave it the following html Anchor Text:

<br /><h3 class="nav-sub-title"><span style="margin-left: -30px">Metro Area Quick</span><br /><span style="margin-left: -30px">Home Searches</span></h3>

The <br /> at the beginning separates it from the previous nav group.

The class and style code gives it the same basic style as other side nav groups, at least on my site. Yours may be different, and may need different margins and possibly other style settings.

I found that the standard REW nav-sub-title class doesn't, by itself, support the proper margin settings, especially for two-line nav group names, which I needed for this example, so I had to force a manual line break and apply the margin style settings to each line individually.

You may find that moving around in the Anchor Text box is a bit problematic with this much code, so you may want to do your special code editing in Notepad, then paste it into the Anchor Text box for testing.