Were 蒸 and 祯 homonyms?
An excellent resource for answering questions like this is 漢字古今音資料庫 at http://xiaoxue.iis.sinica.edu.tw/ccr/.  Unfortunately it doesn't have Song or Yuan reconstructions, but it does list the 攝, which is a Song era classification. 祯 was in 梗攝, and 蒸 was in 曾攝 so they shouldn't have been complete homonyms.  The difference, however, was probably in the vowel, not the final nasal -ng.  The modern zhen for 祯 is a post Song change; some of the 梗 words kept -ng, some changed to -n, but it's unclear when or how it happened.