Redirect if URL does not match
Try this:

# only if a particular domain
RewriteCond %{HTTP_HOST} ^mysite\.domain\.org$ [NC]
# rewrite the URL
RewriteRule ^((?!Library).*)$ http://otherdomain.org/$1 [R=301,L]


The (?!Library) should look ahead, and the match should fail if "Library" is present. Hope this works for you.