How can I reversibly uglify a bike? I'm likely to have to commute to somewhere that's less than ideal for security.  The whole thing is less than ideal but it's only for a couple of weeks. 

Using using the beater bike I already have isn't an option: it's 23&nbsp;km under time pressure,  and sometimes I'll have to use the hybrid with child seat (and try to get there in 55&nbsp;minutes).  I'd rather use the tourer if I don't need the child seat.  In either case I'll take two locks (probably one D-lock and one cable lock, possibly 2 D-locks).  There are some fairly solid iron railings to lock up to, and both bikes are already security marked and registered.

So as an added detterent I'd like to make the bikes look as undesirable as possible -- how can this be done in a reversible way?  While Reduce Resale Value to Avoid Theft? (Uglify) is closely related, the answers there concentrate on permanent approaches, and I'm specifically after something more temporary.
I like stickers. Lots of stickers. Almost anything will do, promo stickers after often cheap and fairly easily available. First put reflective tape on the key areas of your bike, then cover as much of the rest of it as you can with stickers. They will inevitably fade and degrade, but you can help that along by spraying muddy water on them then over-spraying with dilute varnish once the water dries if you want (use acetone specifically to damage the printing if you can).

You can also often downgrade components cheaply - with drop bars hit the bargain bin and buy some cheap, thin bar tape and wrap that over your existing stuff. If you're lucky you'll be able to get two mismatched rolls :) And look out for wrecked bikes and swipe components off them. Rusty, bent seat clamp mounted reflectors, for example. The goal is obvious things that don't affect how the bike actually works.

You could also use packing tape to strap an old torch to the handlebars. Don't use it as a light, it's there because it's big and ugly.

All of that will come off fairly quickly using a plastic scraper and a craft knife, but it'll look convincingly ugly.
For security, I would use secure wheel and saddle attachments (like Pitlock) because that way you can use a grungy old lock around the frame like all the other beater bikes do. Spray paint them black before installing, and don't be too fussy about it. Then get a half decent chain lock (secure, heavy) or cable lock (lighter, not as secure) and thread an old inner tube over it. That way it looks cheap and awful.