Lock contention will be a bitch past a certain number of processes/threads accessing the DB. Also, hopefully your application doesn't crash during the long-running work. If so, your messages are gone forever.
As for a simpler idea, message queues aren't that difficult. Why not just install a proper one to start off with.
If they use MyISAM as the table format for the queue, it may crash and require repairs. If they use InnoDB, then the queue is no longer very lightweight.
Queues in MySQL certainly can work, and are pretty straightforward, but they'll need to be replaced with something else eventually.
"The right tool for the right job" and all that. I just wish AMQP didn't seem quite so sketchy ...