If the Pending is defined as "in a US Government", not just USCIS, it may count the lab certification processing time as the pending time.
A good foundation is that, the priority date is from the lab certification. So it is valuable to check the law and Acts.
-What I said is not based on USCIS, but based on the Child Status Protection Act. Oviously the PERM (Labor Certificate) is not the immigration petition though some immigration petitions need this document such as some kind of the employer-sponsored I-140. The employer-sponsor I-140 needs this document just because the Dept. of Labor want to prevent the employer from not hiring qualified US citizen, and does not means the PERM is part of the immigration petition. This is just like you need your birth certificate for I-485 filing, but the process of your preparing for the birth certificate is not part of your I-485 filing.
PERM filing date as priority date is just for being fair to the beneficial. Because PERM approval wastes a lot of time resulting the I-140 filing is delay, this is not fair to the beneficial (other category such as NIW can file I-140 immedaitely), that is why USCIS use the PERM filing date as the prirority date, and nothing more than that.
Of course, not hurt you to try, but I don't think it will work because the law is very clear there.
