As you say the FCOM/FCTM does not specify a standard callout in reference to checking the PFD/ND for any changes made to the FCU/MCDU. This question was subject to a technical request to Airbus where I work and the answer from Airbus is as follows;
"We do confirm that any action performed on the FCU and MCDU must be checked. A callout is an appropriate way to perform this check.
Any change of modes of the FMA must be announced. Automatic speed target modification is not a change of mode of the FMA. However, such modification can be announced as it would improve the situational awareness"
The way this has been interpreted is "SPEED 160 BLUE" (after selecting speed to 160) or when changing from NAV to HDG "HDG 240 BLUE" etc.
I'm sure there are many different Airline specific methods of tackling this part of the SOP. I have heard of some airlines that have completely dispensed with all flight guidance related callouts. Not sure how that is working out for them.