Very good question… maybe @Nanja can say if this can be done?

You can request a pass cover number here. Additionally, the Interrail reservation service seems to be working again.

Too late, I’ve canceled my Interrail-Pass and by a regular ticket.
Very upset by bad customer service or better to say no service at all.
May you can save some money for Interrail coparing to regular traveling, but I’ve spend 3 evenings trying to make a seat reservation for aTGV, but failed for all possible ways to booking. It’s not worth the time and frustrated me, while I’m usually looking pofitively forward to my vacation!

good bye!

Hi M_M, I've just replied to your private message to provide you with a Pass Cover number but unfortunately I see you've cancelled your ticket. 
I'm sorry to hear about your experience with our Customer Service. I will pass your feedback along.
Some reservations are indeed harder to book, so I understand your frustrations in regards to booking your seat reservation. We are looking for ways to improve but we do have a dependency on the local carriers there. 
I Wish you a great trip ahead and hopefully you will give it another shot to travel with Interrail in the future. 
Cheers, 

@rvdborgt unfortunately we can't send these Pass Cover numbers by default. We are not handing them out as a regular thing. Some mobile Pass holders do not need the number to make reservations so it's an exception for the ones that do have to book certain reservations where it is required. The Pass Cover number is still an in between solution while carriers are adapting to our mobile Passes. We are working towards a final solution with them but given the complexity and requirements from various carriers, it takes time.