Unfortunately it doesn't seem to me that this is possible (or at least, it's going to be very difficult). Although the post found by filo01 shows a way to send MIDI Start, Stop, and clock from the SDrum, The RC-300 manual (page 41) states:
The RC-300 will not receive MIDI clock or MIDI start/stop data from an external device. (The RC-300 cannot synchronize as the slave.)
(This is despite the fact that it seems possible to sync one RC-300 to another - this may be done through system exclusive messages).
In order to slave the RC-300 to the Sdrum I need a way to send a MIDI CC#1-31 or CC#64-95 message
...but the manual - (page 23) only talks about being able to assign those as a way to start/stop the unit, they don't talk about keeping it in sync - and in any case, the SDrum isn't able to send those CC messages.
Unless you have information beyond this, I'd suggest using bits of gear that are supposed to be compatible with each other.