Constructive & Helpful!
It's a neat idea, but if it was to be a thing, it should neither show on character pages, nor ingame for other people, it could give the wrong impressions of other players, especially on character pages, because it would cause some very awkward scenarios when you look through someone's char page, only to find that the item they have equipped isn't actually in their inventory. Maybe if it had some sort of tag that said "test version" or something along those lines, but it still couldn't be showable in your char page inventory though, because it still isn't in your inventory.
I'm no expert in the in depth code of AQW and what is possible and what isn't, but I don't think there's anything imlemented in the game that makes you able to equip a litterally non exisiting item, or an item that does not reside in your inventory.
It will also need to be bugfree and inexploitable when fully implemented, else you can get some pretty nasty scenarios, and you know there's gonna be a couple people who will spread the words of an exploit, if there exists one.
Also, it should only be "testable" if you meet all requirements to buy that item (this is just an opinion though), it may sound a little stupid, because what if an item requires rep and you want to see how it looks? Well, if you like the look of something and it requires x rank in x rep, you will probably rep for it anyways.
If the item is just gold and requires nothing special whatsoever, then this isn't the biggest deal, unless you are low on gold. In fact it would be nice when you are on those "trying to find anything from a shop that fits my new set", so instead of using up some of my gold for something I may very likely sell a couple minutes later, because it didn't match anyways, I would be able to do it without spending gold. Not that gold is generally a problem for any person who farms regularly, but it would still be nice.
If the item costs ACs or is in a shop with special requirements however, is when these rules start to matter, also it will make AC item purchases more enjoyable all round, because you can get a preview of a specific set with the AC item on, instead of just imagining it in your mind. This is really what the main purpose is, I would imagine.
In conclusion, it will need to be perfectly implemented, it will probably take a while to figure out and can possibly cause some scenarios that AE does not want to deal with, they already have enough as is.
If all this was not a problem, then yea, I vote for it, but the risk just seems pretty high.
It can potentially* case some issues, loss of ACs, exploits to avoid AC spending on items and other things, but something that would be pretty nice to have in the end.
*Maybe it isn't a problem, I'm just thinking worst case scenario here.