I kinda doubt Ian McDiarmid has ever recorded anything like that. The best you could hope for is a foreign dub for a language that not many people know.
To me it sounds like an idea that would work better in written form. Otherwise, the viewer wouldn’t be told what language they are listening to (Sith).
EDIT: One idea I just had that is very similar to your idea is how the novel constantly refers to how mechanized his voice is. Perhaps we could apply a filter to his voice to make it sound more robotic? This would disguise it as well.
I just came up with an idea, Kylo murmurs something along the lines of “the Sith language…” after Palps says “At last…” in Sith. Sounds a little… far fetched, but it’s the best I got.
Also, I feel Palps speaking in Sith is a better idea than applying a robotic voice filter to his regular dialogue, it reinforces his knowledge of the Sith.
(CONTEXT, for those wondering as to what I’m talking about)