mysqldump inconsistencies
First, I'll explain why DDL cannot execute during backup. The reason is that MySQL stores the definitions of relations (tables, indexes, etc) to MyISAM tables located at mysql subdirectory in version 5.6 or earlier (, therefore DDL should be prevented to execute during backup).

Next, I'll answer your question. I assume that you use only innodb tables. (If you use any MyISAM tables, --single-transaction option does not act to your wish.) In this case, the answer of your question is "Yes, you can create temporary tables during backup",  because the definition of temporary tables does not need to store  the mysql subdirectory,  and  creating temporary tables is a normal operation under transaction processing.

In other words, there is no problem because definitions and contents of temporary tables are not backed up.