In general the designators prefixes make use of only one letter, and unless space is so critical on PCB, I think the best thing to do would be to reduce the size of the font size on the silk layer. By the way, in rare exceptions, regardless of whether or not it was a correct practice, I had to rename one component or another to replace it with another one that had fewer digits. Another technique I used long before SMD took place was to fit the designators underneath the component, that is, within the footprint of the component, and thus any further maintenance process should be done with the printed drawing of the board layout in hands; another option was to switch as not-visible an array of components with sequential numbering, and to insert its values in the form of a label (I admit, a bad practice). In short, each critical situation requires a particular solution, and the context is that it will determine how acceptable it is.