I learned something today Thanks !
Also if I add \makeatletter or \makeatother in the command of the OP, it will work? Don't really know which one to use (can't test either right know, that's why I ask).
Your command was very helpful, but did not work for me. Here is the error message I get (my file is called backcover.tex): (./backcover.tex ! Missing number, treated as zero. <to be read again> \unhbox l.3 \cleartoleftpage
I found an other version of \cleartoleftpage here ...