I think the MCL round sensor that Peter is thinking of is the wall charger for the Beolink 7000. The standard round IR sensors and the rectangular MCL sensors are very different electrically, so I don't think you can modernise the Xtra speaker kit with a round IR.
The MCL transceiver has a microprocessor which determines which Infrared codes are fowarded to the MCL link.
This micro is what determines the option setting for the Xtra speaker kit, and it is not present in the newer ML round sensor.
With beolink passive and beolink active, the control micro is in the Black box itself - this also supervises the buttons on the round sensor.
The Beolink 7000 wall charger has the same circuitry as the MCL transceiver, just in a different shape. If you can find one of these it will work, however as Peter says they are pretty rare items.
I believe a forum member has connected a new round sensor to the MCL bus, and the infrared receiver works for them. This is not really ideal though, as the timer and mute buttons are no longer available for that room. Also all B&O remote codes would be sent to the Beomaster with no filtering by the front end micro - this could lead to unpredictable operation of the central room.
Whew! Sorry for the long winded reply!