kmark101 wrote: At least, would it be possible to have a small popup window informing the player about the bad character/class selection?
That functionality would have to already exist in the client, either by including a custom message in the packet sent from the server (very unlikely due to localization) or by specifying some existing "That class/race combination is unavailable." dialog. Given that the client restricts race/class combinations by disabling buttons rather than showing any sort of popup, it's unlikely that this could be done without modifying the client otherwise they likely already would have done this since sending a packet is easy.
I know little about the Uthgard client, but the Uthgard devs likely modified the client. Client modifications aren't impossible, they're just expensive since you have to understand the client's assembly rather than writing C# on the server. It's a different skillset and it's generally more difficult and error-prone.