You could configure your apache with rewrite rule as above:
ErrorDocument 404 http://www.domain.com/customerrors/page-404.html
# or maybe .php if you want some smarts in that page?
but also put a system-wide alias in place for the /customerrors/ path
Alias /customerrors /var/www/html/custom-errors/
<Directory /var/www/html/custom-errors/>
# make sure the location can be used to serve files
Order allow,deny
Allow from all
</Directory>
You'll just have to be a little careful to not override a website-local URL.