First, have all Merseyrail trains skip Lime Street. Then, close all Lime Street entrances and exits, bar the Merseyrail station access tunnel. Then, split the tunnel down the middle, and you have a long corridor to funnel people into a somewhat orderly queue. Monitor appropriately
As much as I can see your logic in having people queue in the tunnel/long corridor (I had the same thoughts at one point), It wouldn't work. The entrances onto St Georges Place aren't big enough for the numbers of passengers wanting to join the queue and in an emergency, would just be a crush. That's before you look at the fact the stream of people wanting to go up onto the train would only be able to move as fast as the escalators go which itself would cause issues.
What the tunnel/corridor could do however, is if they made the St Georges Place entrances fully accessible by removing the small steps, is it could act as the booked assistance entrance. Have staff down there to verify that you have booked, if so, you can go in and use either the escalators or lift to get up onto the concourse. This is a low enough number of passengers that the tunnel/corridor could cope with, but also provides a dedicated assistance entrance away from the main queues.
As much as I agree with comments made by others about destination queuing, how many queues would Lime Street need? Cardiff I know it works but their services are much simpler in that most services run all stops, especially on the valleys stuff. Then mainline stuff all trains serve mostly the same stops. There aren't many stations which are served only by one train in that direction. At Liverpool, there are a silly amount of different stopping patterns and so you would end up with some stations being in multiple queues and needing multiple queues for the same direction.
Purely for example, towards Runcorn, you have LNWR and TFW which serve South Parkway, all serve Runcorn, but then beyond Runcorn, Avantis next stop is Crewe, LNWR is Winsford/Acton Bridge/Hartford, and then TFW off to Chester via Frodsham and Helsby. Therefore, just going towards Runcorn suddenly is 4 queues, 1 for each LNWR and TFW to serve exclusive stations, plus then a Runcorn queue for pax who can go on any TFW/LNWR/Avanti, and a Crewe 'fast' queue for pax who can be put onto Avanti or LNWR (no point in merging the Crewe and Runcorn queues else you risk people for Crewe ending up on a TFW service).
Towards Warrington Central you have TPE with one stopping pattern, EMR with another stopping pattern, plus then Northern with 2 different stopping patterns. Plus a number of stations which fit into multiple of these service patterns. You get the idea.
Cardiff is much easier to plan these things because most services are all stops, and there are good interchange points not too far out for regular connections onto other services (ie a Barry/VOG queue people can get on the first train out then wait at Barry if they specifically want Barry Island or VOG. Treherbert, Aberdare, Merthyr similarly at Pontypridd. Rhymney line if the train finishes at Caerphilly, you just wait at Caerphilly and by the time the Rhymney train arrives, it should have dropped off enough people on the way that you aren't full). Liverpools mix of fast and slow trains and various stopping patterns, as well as poor potential for interchanges at outer stations (you wouldn't make someone get on the Avanti to Runcorn to then change onto TFW to Frodsham).
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