This is an old thread, but I thought I should post something similar I did.
I used RadToolTip to display error messages for ASP.Net validators as in following video: RadToolTipify ASP.Net Validators.
In my page all ASP.Net validators show their error messages in a RadToolTip; even the validators with no controltovalidate set will automatically show a RadToolTip next to a target element mentioned by user or next to the clicked button. I found this to work for custom validators as well as standard validators.
My hosting provider has been down for whole day, but once its up and running I will post a demo page and paste the demo link here.