Read up on the design of the Drake TR-3 SSB/CW transceiver. This radio had a rinky dink pair of 4 pole crystal filters which are affectionately called the "soup can". This was an Achillies heel of this radio and often one or both of the filters went south.
The topology of this radio used a fixed BFO frequency of 9 MHz, and the two filters center frequencies are such where one is above the BFO and one below he BFO. To switch sidebands, you switched the filters.
These filters today are unobtanium and when one dies your TR-3 is dead! The TR-4 uses the same approach but employs two separate filters not soldered inside a soup can. The TR-4 filters are physically too big for a retrofit inside the TR-3.
But an ingenious solution from K9SUL who had one dead filter (the other was OK) was to hardwire that filter into the circuit, bypassing the front panel filter switch. The next step was a technology solution. He eliminates the 9 MHz crystal BFO and installs a Si535i driven with a Teensy Microcontroller. The front panel switch then selects a 9 MHz BFO frequency for the one good filter and a second BFO Frequency that shifts the BFO signal to the opposite sideband.
I foresee a twizzle on this approach where a single 9 MHz 6 or 8 pole filter is installed in place of the soup can and the Si5351 would then select the proper USB LSB carrier oscillator frequencies. You could even resort to a 4 pole 9 MHz INRAD filter which costs $33 and has a Zin/out of 200 Ohms. You will need a 10 to 1 match for this filter as I believe the soup cans ones were 2K. A 6 Turn to 19 Turn transformer on a FT-37-43 core gives that transform. 6^2 = 36 and 19^2 = 361. Thus 361/36 = 10:1.
This not only rescues a boat anchor from the land fill, but a 6 or 8 pole filter would be an upgrade.
Yet another use of the Si5351. K9SUL has a you tube video on his solution.
73's
Pete N6QW