Since "This is a light-hearted question", I need to warn you:
If you're asking for an "ungetoasteter Toast" you need to be careful about the proper pronounciation of the German "u". If you pronounce it like the "u" in English "untoasted", you'll still get a lightly toasted toast, because your German host will make you an "angetoasteten toast", which was started to be toasted but not quite finished. A German host producing a properly "angetoasteten" toast will make sure, there's still lots of white and just some yellow-golden tan about the rim of the toast. The proper technique is to toast mere seconds and rather ask, if that's enough than to risk any brown spots.
Otherwise you might ask for "eine Scheibe Toastbrot". The perceptive host will ask you, if you want it toasted. Only the toasted toast is commonly referred to as "Toast", e.g. "Toast Hawaii" is a dish prepared with toasted toast, ham, ananas and cheese. Asking for "Toastbrot" specifically should be uncommon enough to make most people wonder and thus ask.
However very well-meaning (and/or care-free) hosts might want to please you and toast it without asking.
Asking for Weißbrot might get you sweetened bread possibly with raisins. It would definitely not be toasted though.
Finally asking for "Sandwichbrot" should be safe except for the most insistent toasting hosts.