Yes, and no. While is is not possible to create a redirecting "record" as such, it IS possible to get around the problem quite easily.
This is easiest if you have a static IP. In this situation, all you have to do is create 2 webhops, one for yoursite.com, and one for www.yoursite.com, each directing to the url http://xxx.xxx.xxx.xxx:81/ , where xxx.xxx.xxx.xxx is your Static IP Address.
If you have several sites like this, one possible workaround is to use a different subfolder for each website. A better workaround, however, is this next one:
This is most certainly possible to do with a small amount of work - this is how you would do it - we will end up with 3 DNS entries - yoursite.com, www.yoursite.com and www2.yoursite.com.
Like the previous option, yoursite.com and www.yoursite.com will both be webhops, but instead of pointing to your IP, they will instead point to http://www2.yoursite.com:81/. Create those two entries.
Next, create a dynamic domain www2.yoursite.com.
Now, when you use the dynamic update script or program to update your IP, it will update www2.yoursite.com, which will in turn make yoursite.com and www.yoursite.com work!
Important note, if you use VHosts on your web server:
On your server software, if you use vhosts, you will need to add a vhost entry for www2.yoursite.com, as well as www.yoursite.com - the server will see the request coming to www2 instead and redirect accordingly.