Some ideas I came also across when I had to transmit efficiently small and large amounts of information
1) use a 9bit UART and put start of frame in the 9th bit, then you have a very easy way to syncronize
2) use the 8.bit of the first byte to switch between large data including length with the rest (7bit) or small transport using the rest of the 7bit as selector of fixed data types.

Or.... just use your phantasie

Werner

PS the bad thing about using the 9. bit is, that I've found so far no Uart SW for the PC that can handle this correctly when debugging is done via PC