My previous solution was clean in the sense that it voids active waits. If you don't bother having slightly active waits, you could keep a queue file in each directory, with write access protected via a lock, in which you append your 'name' (e.g. pid number), then keep controlling until it's your turn. When you're done, you simply remove yourself from the top of the file (using the same locking to prevent multiple writings). In this way, the access to the queue file should be rather quick, then a check every second should suffice without slowing your machine to a crawl.

Note that if you use PIDs, you can also check if the current first-in-the-list is alive, just to prevent infinite locking by a badly dead script.

There is also a problem if the reading script stalls and requests continue to be written, overflowing the FIFO's buffer. Nonetheless it would normally work and if the commands are short a lot of them would fit into 4kB or whatever size the FIFO buffer is.

No, thats not a problem because every program trying to overflow the FIFO will stall itself and give enough CPU to the stalled script to empty the buffer. The kernel sees to that.