Will the RTC be compatible to the RTC of the LS3?
The answer is Yes and No:
Yes: the Speak&SID firmware is open source, so if you wish to customize it such that it provides, for I2C, the same clock functions that LS3 does, then you can do that with not so much effort. This, however, results in a I2C / SPI-device specific API, which is not generic. Hence, the answer should really be:
No: the idea for the SPeak&SID card really is to provide "generic" API functions for I2C and SPI to the CPC in the firmware. That means, rather than having I2C or SPI-device specific API functions such as "set hours", "get time", etc., let the CPC construct the right I2C / SPI message instead, and also do the decoding of the corresponding responses received from these devices. The Speak&SID firmware will only provide generic low level I2C / SPI functionality - buffering of input and output message, and send and receive.
So, "Yes" will require that you put the high level SPI/I2C-device specific functions such as "set hours / get hours" into the AVR firmware yourself, and you can make it any way you want.
And "No" will imply that you will have to wait for me to provide these generic I2C / SPI API functions, and then the device-specific code (set hours, get hours, ...) will be dealt with by the CPC.
It should be noted that the latter approach will only work for "simple enough" devices, but not for very complex ones (i.e., it is impossible to set up the Epson speech chip like this for example).