[triodan]

It's a combination of two existing characters, the rei as in meirei/命令 (order, command) and wa as in heiwa/平和 (peace, harmony). No new characters need to be reserved.

[qlk1123]

Also, this seems to be the first time that Japanese Royal choose the kanjis from their classical literature[1] instead of Chinese literature.

[1] https://en.wikipedia.org/wiki/Man%27y%C5%8Dsh%C5%AB

[tanilama]

Rei, 令, in this case means 'good, fortunate', not command.

[archgoon]

Huh, according to Jim Breen's dictionary (http://nihongo.monash.edu/cgi-bin/wwwjdic) 'good, fortunate' is '礼' (rei). '令' (rei) is listed as 'command, decree'. The official character that is being used is '令'.

Is there some rule or convention that explains why 令 means 'good, fortunate' here? Is it due to the usage in the 7th century poetry that is referenced in the article?

[ekianjo]

Unless you do mind reading this is all a matter of interpretation. There is no single meaning.