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There is at least one document on the live Web that uses an ASCII-compatible encoding that has ç at code point 0x87: http://memory.loc.gov:8081/ll/llhb/030/llhb030.txt (Search for "07390000" in that file to get close to the example.) One encoding that fits this description is IBM Code Page 850, which is supported in Firefox (as IBM-850), but does not appear to be in the Encoding spec.
Another possibility is IBM Code Page 437, which does not appear to be supported in Firefox.
Since the IBM encodings were not interoperable we decided to not have them (with ibm866 as notable exception).