Are there any restrictions, in terms of length, ability to include non-ASCII characters, etc. on the name of a Postgres column alias? And have there been any changes to such restrictions from version 8.1 to the present?
The rules for a column alias are no different to than those for regular column names.
http://www.postgresql.org/docs/current/static/sql-syntax-lexical.html#SQL-SYNTAX-IDENTIFIERS
SQL identifiers and key words must begin with a letter (a-z, but also letters with diacritical marks and non-Latin letters) or an underscore (_). Subsequent characters in an identifier or key word can be letters, underscores, digits (0-9), or dollar signs ($). Key words and unquoted identifiers are case insensitive.
There is a second kind of identifier: the delimited identifier or quoted identifier. It is formed by enclosing an arbitrary sequence of characters in double-quotes ("). Quoted identifiers can contain any character, except the character with code zero.