Jonathan, If you want to remove the padding from, let's say, month and hour it can be done with the "FM" prefix:
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spi_exec "select to_char(''$1''::timestamp, ''<b>FM</b>Month DD YYYY <b>FM</b>HH:MI AM TZ'') as s"
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This works in PG 7.1.3 and beyond. (Maybe before that too)...