I'm pretty sure the tunnel rules, even before the changes to allow DB, would have allowed NOL sets to run through in passenger service. The rules would have been written with NOL sets in mind; after all it is what the NOL sets were built to do.
The NoL sets would have been going through the Channel Tunnel in pairs, not as standalone units, which would have meant that they would be no different in size then to the full size 373 units.
You see what would have happened is a set from Glasgow/Edingburgh would have joined up with a set from say Manchester at somewhere like Ebbsfleet say joined together and then gone on from there coupled together.
What DBS are planning though is different in the fact of you will have two IE4 sets joined together leaving Frankfurt, but just over the German Border one set will go to say Brussels or Paris, with the other set continuing on to London. The IE4 set would be one it's own, where as with the NoL sets working coupled, you could have at least one set doing the recovery if the other set failed within the tunnel.
As Peter has stated above these rules have recently changed, which will allow the DBS IE4's to run through as standalone trains to the UK.