Can you expand her "mess-proof" space beyond the bathroom but not so far as the kitchen? For example, our bathroom is off of the master bedroom. If I'm showering, my kids can be in the bathroom with me or in the master bedroom or their bedrooms. In my bedroom I turn on the TV or video. This works for us because we have a gate on the top of the stairs, so it gives them enough space to keep busy and amused, but doesn't give access to dangerous (for many different reasons) parts of the house. Take a look at your house and see if you can design some similar space somehow. My 4 1/2 YO cannot open the wall-mounted gate at the top of the stairs, so maybe you could find something similar to use to block access.
A practical note: Just because its a mess doesn't mean it has to be cleaned up right away, unless its dangerous or will get much worse while you are out (probably depends on where your child is when you are gone). I have been known to walk away from some real major messes because I didn't have time for them. Yes, dealing with them when you get back isn't fun, but it might beat being late.