Psu / grounding

i’m about to build me a metal / wood case for the Shruti-1. I’ll have the boards mechanically decoupled, i.e. all pots, buttons, jacks, leds mounted to the panel. i’ll also get rid of the 1044 switcher by adding a bipolar power supply.
for this purpose i have here a little 2x 9V (2x 266mA) transformer. i will just rectify and prepare the 9V so it can be used by the vregs.

as i’m using a metal case i’d like to ground it. now my question is:
can i take the circuit’s ground (i.e. the transfo’s secondary center tap) and connect it directly to the protective earthing conductor (yellow-green here)?
i believe i remember (!) having read in a tube amp book that this is called protection class 1 which i’m not so sure about anymore…