You'd probably need to check the current implementation in the source code, namely AT91_UART.cpp. Unfortunately, oversampling configuration is not supported in the managed wrappers (SerialPort class). Please note COM1 is in fact Debug Unit, which has limited features (compared to 'full' USART module), as described in the datasheet section 26.4 DBGU UART Operations.
Ok, depending on if you can divide it 10 and the remainder is more than 5 it divides by 10 and adds 1. But it does look like it more or less just passes an int in for the baudrate in hz. I did look at the datasheet for this, but doesn't really help.
At the bottom, there are two functions, BaudrateBoundary and IsBaudrateSupported.
I will try to get something on the scope and we can see whats actually going on then.