You can actually do your 301 rule
like this:
RewriteEngine On
RewriteCond %{HTTP_HOST} ^(www\.)?olddomain\.com$ [NC]
RewriteRule ^ http://newdomain.com%{REQUEST_URI}?redir=1 [R=301,L,NE,QSA]
Now on newdomain.com
page just check for ?redir=1
in URLs, if it is found then show appropriate popup message to user.