The Virgin assessor will never see an SOE book filled out at Qantas. SOE will not be in use anymore.
What the assessor will see is tasks written up in the Virgin OJT book that were completed at Qantas and later transferred from the old SOE book into the Virgin OJT book.
I say again. The books themselves, whether SOE or another organisations OJT, are not transferrable into a particular organisations OJT system, only the tasks contained in them are.
I have never heard of a 75% rule. In the prologue of any OJT book will be instructions on how it is to be used and the percentage of tasks that must be completed, both mandatory and non-mandatory. The percentage required will be related to whether it is a first type rating, second or subsequent, whether POC has been completed, or whether the other category on type is already held. There may be other factors but these are the main ones.
I cant speak for AA but they don't have aircraft type specific practical OJT books approved by CASA so I would be wary of treating as gospel what they have told you.