> Many, but not all (I think Japanese uses some of those hints as well). But if you have "character from some ancient text" that doesn't have that radical you're SOL.
The Japanese pronunciation hints are way less reliable than the Chinese. First, Japan inherited Chinese sounds and characters in drips and drabs. Because Chinese has so many dialects, and a dialect may pronounce a certain character a vastly different way, the onyomi today may be a presentation of old Fujian pronunciation or old Cantonese, not the Mandarin. Second, because the characters were introduced to different parts of Japan at different times, those characters were used for different purposes in the different regions in Japan. Today this means a single character can be used in different words with different meanings, with more than one onyomi. Nevermind what happens in kanji compounds.
If a Chinese character was suddenly introduced with a new radical, that would be akin to suddenly finding a new letter in the alphabet. Highly unlikely. When is the last time a letter was added or removed from the standard English alphabet? A quick google says the last letter added to the English language was J, added 1524. Currently there are 214 radicals in Chinese. A new character would almost certainly be a combination of those existing radicals. Furthermore, the character could still be systematically written in computer language with brush stroke entry. That character would simply have to be added at a unicode level for image rending, same as having to define Alien Emoji as U+1F47D so that it does not render as a ?