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.
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?
https://kotobank.jp/word/%E4%BB%A4%E6%9C%88-660939
If we have to trace back even further, it can date back to the ancient Chinese classic, the Classic of Poetry, where it is being used as good/respectful:
https://baike.baidu.com/item/%E4%BB%A4%E9%97%BB%E4%BB%A4%E6%...
https://mobile.twitter.com/nick_kapur/status/111270395164693...