You should be able to go under Firewall>IP Based ACL and create your allow policies for the two IP addresses associated with the allowed URLs. You can then create a deny statement for all other traffic.
If the IP addresses change then you will not be able to access the URLs since they would no longer match the allow statements. By default using the ACL is the only way that I can see that you can lock down all websites except just a few. I do not have a router with Protectlink enabled for me to test whether or not it is possible to using Protectlink but I believe that it works mainly using URL reputation. Someone who is more familiar with Protectlink maybe able to address this for you.
I believe that you can get evaulation keys for Protectlink to demo the service and see if it would fit your needs.
Being a person who likes to save a Dollar when i can. Check and see if you can make use of the Firewall> internet Access Policy tab.
if I'm not mistaked, in Firewall> internet Access Policy tab, the only thing I can do is add which url are forbidden. I can add up to 20 url by domain or keyword, but I can't chose blocking all domains and permit x domains. As result now I'm blocking "hotmail", "gmail", "msn", "facebook", etc. since 20, but all other domains that I haven't add in the list, are permited.
I hope you understand my english.
Your english is very good. I did say "check and see" in my earlier post and not here is a solution.
To answer your original question , the answer is NO.
URL filtering is a limitation you will find in a product like the WRVS4400N, but you will not find that limitation in the new and more powerful SA520 which has an URL allowed and blocked list.
My advice would be, evaluate trendmico protectlink if you wish and see what you think. send a email to the following address to get a evaluation license;
But try a SA500, if that customer really needs a low cost, but capable router that requires URL allowed list.
Really I'm very glad with WRSV4400N, I think it's an amazing router firewall, it's almost perfect for my customers kind. Perhaps I will try SA500.
Thank you very much.