I need to learn how to install a wall hung lavatory on wholly plaster wall. Is there any chance to mount the lav without any plate or carrier? What do you advise?
You CANNOT do it safely. But, if it is all you have available, you can use toggle bolts for most of the holes if you can get one or two screws into a stud.
Why don't you want to install the wall bracket? And when you mention "carrier" that is a whole different ballgame. If it is a sink which requires a carrier, there is no option.
I don't want to cause any damage on wall. The lav is 50 cm lav. An additional equipment will increase the lav costs to customer. I just wonder if I could do it without any kind of frame on plaster wall..
Ask your customer if they would prefer to pay now for the additional equipment to support it correctly, or if they'd rather pay MUCH, MUCH MORE later when the lavatory falls off (remember to factor in the cost of the damaged wall and water damage to the property).