I use the KeePass AutoType feature to log in to various RDP sessions in the network.
On the Auto-Type tab I add a custom sequence for a specific window.
To add the entry I use the default sequence but add to the beginning of it. This means that it doesn't matter what username is currently displayed in the security details as the sequence forces it to use the "Use another account" option.
To set up the custom entry you need to have the Windows security dialog open and waiting for input then go into the KeePass Auto-Type tab and press Add then fill in the box as shown:
Works for me!