I don't agree that each replacement prefix would have to be priced separately. The value of radar notes as they are shown in the catalogue is explained by this statement on page 338:
"Full prices for the common signatures and prefixes are given. They are not intended to represent premiums to be added. Reasonable allowances must be made when pricing more valuable varieties, or notes in Choice or Gem Uncirculated condition."
So a separate table for replacements would be feasible. There is no golden rule saying that all special numbers must fall in line and be represented equally. Radars are still the most popular (or at least the most ubiquitous) kind of special number and they should be first in line when given special consideration. However, I would agree that it's NOT a good idea. My reason is that the majority of people don't know how to use the back-of-the-catalogue tables. They don't "get" the statement I quoted above. If the value of their specific note is not printed exactly in black-and-white, most people don't know what to do. As much as the catalogue editor tells us that the prices in Charlton are based on the market which is dynamic and changing, most catalogue uses make Charlton their Bible and are unable to comprehend how a note could trade for anything other than the number printed in the book. In other words, they think the market is based on Charlton, not the other way around.
I'm also concerned that Charlton's initial attempts to price radar-replacements would seriously undermine the market. In other words, if I have a note for which I want $100 and Charlton comes out and tells us the note is only worth $30, well, that sucks.