I'd like to move this thread into the Alternate Fuels area if you don't mind?
Normally the LPG computers piggyback onto the OEM ECU, so you keep your existing ignition, and the LPG ECU just turns on its injectors when you're on gas and turns off the petrol ones. Then vice versa when you're on petrol.
Maybe you're saying you'd like to recurve the ignition to "optimise" it for gas, then switch back to the standard map for petrol? If this is what you're after, I don't think it's possible because it's hard coded in the Lexus ECU.
There might be some clever boxes out there that could "fool" the OEM ECU by introducing delays in the Ne, G1 or G2 signals, but that would be dodgy....