While the HS2 plans are for 200m classic-compatibles to get to Glasgow, it would be absurd to not consider future possibilities when rebuilding the station. Now that the plans are for captive gauge platforms to be at 1155mm above rail height, they become much more desirable in themselves for HS2 services as they'll permit complete level access and possibly even platform edge doors. Today, the long platforms at Central have curves in them which make sighting and access difficult for all. Any station in GB where it's practical to provide dedicated 400m platforms for HSR services will likely seriously consider setting them up to be captive TSI standard, even if they're completely marooned from the rest of the TSI gauge network.
Remember that as the rail network develops, we'll see specialisation of tracks and platforms. In future, Central will probably see most of its suburban services sent onto the metro network or into a tunnel, leaving only longer-distance regional and LDHS services left. With the plans for the bypass line to Rutherglen, the easternmost pair of tracks into Central station will end up used only by LDHS and express services to Edinburgh, which could all be designed to handle the captive platforms. Therefore it's not as unreasonable as it sounds. In day to day service a Stranraer bi-mode rural train won't have any good reason to cross over all the way to the LDHS platforms, and then find the captive gauge to be problematic.