Worth pointing out that the pass number that you enter when you book a Eurostar reservation doesn't actually appear on the Eurostar ticket anywhere. There's no connection so as long as you could produce a valid Interrail pass along with your Eurostar passholder ticket if asked (I never have been), to prove your entitlement to the passholder fare, then that would be fine.
I just did a dummy booking on Rail Europe using the pass number (6 digit, alphanumeric) for a mobile pass I've bought which I intend to use in July. Validated.
I tried the same booking but using the number from a pass that expired last month. Not valid.
However... I used the Pass Cover Number generator on the Interrail/Eurail web site to generate a Pass Cover Number from the number of the expired pass. I put this Pass Cover Number into Rail Europe. Validated.
I then used the same generator with a completely fictitious pass number to generate a Pass Cover Number and put this into Rail Europe. Validated!
I then just entered AAAAAA into the generator and used the Pass Cover Number that it gave me on Rail Europe. Validated!
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So it appears that Rail Europe can identify an expired/invalid pass number, but not so much with a Pass Cover Number. Either way, once you've managed to buy the passholder fare, Eurostar aren't going to care what number you used to buy it.