I suspect more than a few syntax adjustments would be required before users could rely on the port. The upstream project (e.g. wordpress) would need to be aware that their users were trying to use postgres in order for such users to be actually happy.
I could be wrong, but I have not seen a port of a real app that came down to just syntax differences.
It might be a different story between two implementations that follow the SQL standard reasonably well (MySQL is pretty far from the standard in many ways), but even in that case I'd be cautious.
Postgres is doing great being postgres. To the extent postgres can accommodate converts from mysql, or oracle or sql server, then also great. But we're too busy inventing new stuff to endlessly chase the quirks of other systems.