I think SPREE and Governance seem like the logical approaches to this problem if the parachain is completely inaccessible.
However, I think probably the best goal here is to ensure that on-demand parachains can always continue to live as long as there are willing collators for the on-demand parachains. This basically means we should ensure that the on-demand parachains do not have some kind of fixed collator set, or at least has the ability to transition to an open collator set on its own. (for example, bypass the collator set if a block has not been produced in 7 days)
I briefly talk about some ideas for this here: On-demand parachains - #13 by shawntabrizi
Much like a proof of work chain, as long as an on-demand parachain has an open collator system, it is “unstoppable”.