Did Tolkein bite off of The Wizard of Oz, when Frodo & Samwise disguise themselves as guards?

No. They really had no choice but to disguise themselves or else the orcs would have immediately known they didn't belong there. Especially since they thought Sam was an Elf and thought he might have someone with him, so they were already looking for elves or non-orcs. Plus, the orcs took Frodo's clothes so the only thing he had to wear was clothes from one of the dead orcs. LOTR is so original so I really don't think he would have copied.