I think that the following trick will work:

Program your 8051 serial interface for 8-bit operation.
1) Transmitting from the 8051: Transmit a byte with your 7-bit data and the MSB always set to '1'. The MSB will be interpreted by the receiver as a stop bit. And the 'real' stop bit that follows will only make a longer stop bit.
2) Receiving with the 8051: If bytes are received with some delay between them, there will be at least 1 bit at '1' level (idle) after the stop bit. The real stop bit should be interpreted by the 8051 as MSB=1, while the idle state should be interpreted by the 8051 as a valid stop bit. If the byte stream arrives at full speed (w/o the required idle state between bytes), the 8051 can be override. In this case, program the external device for 2 stop bits, if possible.

Regards,

Alfredo del Rio.