Spanners,
Many of the charter carriers you mention only do their own LINE mx. The heavy checks are farmed out, and by heavy I mean above and including 2A's.
Even those carriers that have their own in house mx will some times realise that they will be unable to c/o certain checks due to, for example, other work in progress. So they too will seek 3rd party mx for some of their fleet.
Whilst I admit that line mx contracts are a bit thin on the ground there is scope for hangar contracts.
There is a lot of potential out there but it is a cut throat industry.