...

This might work, but no guarantees.

OK, so I'd be better off making sure all Haskell runs in one OS-thread.

My concerns still apply - non-thread-friendly library calls may mean that GHC's RTS interacts badly with the rest of your threaded program.

Although presumably I can use Haskell-threads: I can have a special Haskell-thread that handled a queue of incoming execution requests, firing off a new Haskell-thread for each one. And then I could have a native-code mechanism to allow asynchronous additions to the queue.

Yes, there's no restriction on using Haskell threads because they're handled internally by the RTS. Cheers, Simon