Have you taken a look at the /WICED-Smart-SDK/Apps/uart_firmware_upgrade project within SDK 2.2?
Within this project, there is a Hello_Sensor.C file which claims:
* If target system consists of an external MCU connected over UART to
* 20736/7, MCU might want to download a new firmware. This sample shows
* how to add an Upgrade over Peripheral UART feature.
I believe this was not included in SDK 1.1, but should work? A UART driver of some sort would still need to be created on the MCU so that it can communicate with the BLE device.
I'm not aware of any limitations, but other users on the forum have created interfaces from an external MCU, so hopefully they can contribute to the discussion as well.
Porting the WsUartUpgrade application for Windows to their MCU platform would work as well.
Please note that this cannot be used for first-time programming of the device as described here: Re: Production Programming of BCM20737S