I'd have to warn you against it. I've been to US military installations before and since 9/11. Before, it was relatively easy, but now they've really tightened the screws. Showing your MOD 90 might not elicit anything more than a polite '**** off'.
They might recognise it from the British exchange officers, though. If you're really desperate, you could contact just that chap. The current or last issue of the Infantry annual (you know, the one with the bayonet shape for a cover, wasn't it called the Infantryman? I have an old copy here somewhere...) should have his name in it, maybe even e-mail.
Plus, I gather the PX there isn't that great. Camp Bondsteel's a lot closer to home, you know, and so is Butmir - both are better-stocked than most stateside ones.
Generally speaking, Brigade Quartermaster (actiongear.com) and suchlike have a much broader selection of Septic kit at cheaper prices (watch out for sales). Buying electronics in the States is gonna be a lot cheaper in the States - whether you buy them from Radio Shack or the PX. You used to be able to make a killing on stuff like XBox games for a while, but they've started to adjust European prices downwards as the Dollar loses value.
Doh! and lest I forget: Make sure you don't lose your MOD 90 once on base like I did last year. Tried buying Lost in Translation at the friendly video store, noticed my faux pas and was told to shove it, despite my protestations of "How could I have gotten on base without one?!?".