Sadly I don't think it's possible. The way the game handles inventory kinda makes this difficult at best and impossible at worst. You can edit the number if items you have in game using cheat engine or another memory editor however you can't add NEW items that way cause it crashes the game. Editing the save would likely just corrupt it. However using a cheat engine script or game trainer you can breeze through the game as if it was no trouble(with a few exceptional missions which might go awry with said scripts) and get most if not all costumes rather easy. For example SPP(or whatever it's called) is rather easy to hack so you can either max it out or better yet you can maximise the amount each guest character in a mission gives you when they leave thus allowing you to hit those rewards for EARNING SPP. As for the rest I think most of them are rewards for speciffic missions or something so just do all you can. (the day's timer is also easy to manipulat using a memory editor)

I have actually managed to do it. I found the addresses in memory where the costume selection is kept, so by changing those values, the game thinks you have selected a different costume, and switches to it when you leave the save menu. I am going to make a cheat engine table for ease of use, but I am not sure I am allowed to upload and paste it here :P

I wouldn't play through the game by cheating to unlock the costumes, because I am the kind of guy who plays a game just once, and doesn't stick around for just the gameplay.

Yeah I can say from experience that uploading that table here is NOT a good idea. I made a guide specifficaly for the purpose of teaching people how to use CE (Just basics with some links to youtube videos) and that guide got blocked so only I can access it now. I dunno if it was cause of links, because it was referencing cheating and/or memory editing or whether it was cause some a.s.s.h.a.t posted a complaint(selfritious b.a.s.t.a.r.d.s) but it would likely happen here too. So maybe post someplace else then post a direct method to geting to that web page without posting a link. Or post a link and replace www or some of the more recognisable characters with * or something so steam doesn't recognise it as a link.

there is actually a group with links to cheat tables dropbox directly on the group page.

true but it's to dropbox not directly to a cheat table itself and I bet the link doesn't have anything to lable it as a cheat table. Also I wouldn't put it past somebody to report those links and once that's done they will close em off.

Ok so, if anyone wants it they can get by adding me

I'm just saying try an indirect approach. Valve tos only really applies to directly linking to stuff like that but linking to a website to which you have uploaded a direct link shouldn't trigger them. Say for example Put the link in a plain text file from note pad then upload that to dropbox or mega or whatever. The effect is the same but I don't believe it would technically breach their tos so they shouldn't do anything.

Or at least nothing more than delete the link in which case it can simply be reuploaded or broken up like I said previously

not to worry, if anyone at all really wants to have the costumes without unlocking them, they will find this thread eventually.