yes it is possible to have an external feed on a wet sump set up.
I am about to fit an electronic accusump to my RB26 . It isnt exactly what you have drawn but it is close .
This is how my system will work
On initial start-up when the valve on the oil side is opened the pressurized oil is released into the engine and therefore pre-lubricating the engine prior to start-up.
The Accusump holds whatever oil pressure the engine has at the time that it is shut off.
After the engine is started and the oil pump has taken over, oil is pumped back into the Accusump.
This moves the piston back and pressurizes the Accusump until it equalizes with engine's oil pressure.
While driving, if the engine's oil pressure is interrupted for any reason........ (for example this can be set to come in at a prearranged oil pressure drop . (on mine it runs at 7-9 cold and 4-5 normally with tickover at 3 . I would set the valve to work at 2.5 ).........the Accusump releases its oil reserve again, keeping the engine lubricated until the engine's oil pressure comes back to normal.
This release of oil could last from 15 to 60 seconds, depending on the size and speed of the engine.
In racing or hard driving conditions, the Accusump will automatically fill and discharge when needed as you corner, accelerate and brake.