GLA could be done by Loganair Twotters if they have any gaps in their schedule.
Is Loganair at liberty to operate the government-owned Twin Otters for commercial, non-PSO flying? You will be aware of the fact that the Twin Otters no longer have an airline livery, but fly the Saltire.
Loganair do still have a Twin Otter of their own, in addition to the two Saltire branded aircraft that are owned by the Scottish Government.
There is always the Do328 that sits in Dundee for most of the day between Stansted runs. No reason it couldn't position to Glasgow and operated GLA-IOM-GLA as a daily service, opposed to Citywings double daily.