This is a great idea! especially if you're doing something that's fiddly (paper piecing??) and you're pressing contstantly.
Please keep safety in mind, ladies and gents!
Unless your wooden or plastic TV trays are a lot more stable than any I've had or seen, I'd be worried that the iron could go flying all too easily. Fire hazard? or burning yourself, family or pets?
Using plastic for an ironing board, even when it is covered, is more of a risk than I'd want to take.
Please be careful everyone