Ok, yes that will work. I thought you wanted to connect the gate to the junction of R25, and R26, which would not work.
You do however want to leave diode D3 installed, or your DSO will not work if the battery is dead. Also, the voltage available to the DSO with the FET circuit will be less due to the voltage drop of the forward biased diode in the FET (since it will be turned off when charging). There should be no problem leaving D3 in, if the FET circuit is added, and in fact, it would be preferable.