I can confirm RB1100AHx4 works fine with -48V DC power. Better not mix that with PoE-in on ether13 though, CRS328-24P trying to power up (when already powered by -48V DC) reports a short circuit. The same DC/DC converter is probably used by both the DC terminal blocks and PoE-in. Two 230V AC inputs are also available and work as redundant with DC.
Thank you for the confirmation!
I wasn't planning to use PoE-in.
I just need to power RB using existing DC PSU which has positive grounded, and the question was:
Is it safe to ground RB1100AHx4 chassis to existing grounding bar?