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seems its a bit more difficult:
encoding 'macintosh is registered by iana[1] (nice formatted in [2]) and is 
covered by RFC1345[3].

Name: macintosh [RFC1345,KXS2]
MIBenum: 2027
Source: The Unicode Standard ver1.0, ISBN 0-201-56788-1, Oct 1991
Alias: mac
Alias: csMacintosh

[1]http://www.iana.org/assignments/character-sets
[2]http://www.cs.tut.fi/~jkorpela/chars/sorted.html
[3]http://www.faqs.org/rfcs/rfc1345.html

so far the hard facts from specification view. in all these specs are 
mac_roman etc. not mentioned. So what?

I found at [4] with the popular program 'recode' a hint of the alias. the aothor 
there uses the iana registered macintosh as an alias for mac_roman:

DEFENCODING(( "MacRoman",               /* JDK 1.1 */
              /* This is the best table for MACINTOSH. The ones */
              /* in glibc and FreeBSD-iconv are bad quality. */
              "MACINTOSH",              /* IANA */
              "MAC",                    /* IANA */
              "csMacintosh",            /* IANA */
            ),
            mac_roman,
            { mac_roman_mbtowc },         { mac_roman_wctomb, NULL })

[4]http://recode.progiciels-bpi.ca/showfile.html?name=fusion/recode-3.6/
libiconv/encodings.def

Because of that (I trust recode somehow) i would propose to add macintosh 
as an alias for mac_roman.