Sorry for being very late. I also think guc version of the patch can be acceptable and useful.
I modified the patch as such and added to commitfest 2017-07.
You need documentation changes in "libpq - C Library" chapter's PQexec section, where it is mentioned that command string can contain multiple SQL commands and explain how it behaves.
I think GUC's name can be something like "multiple_query_execution" and setting it ON/OFF will be better. I think others will also come up with some suggestions here as the current name doesn't go well with other existing GUCs.
Also, consider adding this new GUC in postgresql.conf.sample file.
