(Julian, if this is off-topic, or felt inappropriate, please delete it. My feelings won't be hurt!)
Frank,
Pardon my stepping into the middle of this, but have you had the opportunity to look into the product VCLSkin? Unless you have a lot of vested interest, time, money, and code into Developer Express Skins Library, you might want to check VCLSkin out!
Like WPTools, it's a native Delphi app, and comes with source code (although I've only had to make one change to it, ever).
I'd researched a number of skinners (including, of course, TBX). It allows you to skin your application(s) with almost NO changes to it - other than dropping its component into your app.
I've used it for a few years quite happily, and the guy definitely keeps the product current, and addresses any problems brought up. He also makes available, at no cost, additional skins from time to time (as well as giving you an editor that lets you create your own, or tweak existing ones). It's not expensive at all.
Regards,
richard diamond
p.s. AND NO, I have ZERO connection with that company, its product, or the developer!!!