Indeed I have, and waaaay back in '86 I worked with a visual inspection platform whose recognizer was built in FORTH. Awesome stuff.
FORTH's flexibility is of a different kind, though. Its flexibility coms from its ease of extensibility, not from syntax flexibility. FORTH and LISP have much in common in that respect, but that's a different thread entirely. I would argue that both of those languages are very rigid in their syntax but also very minimal in syntactic requirements. In other words, there's only one way to write them, but lots of ways to extend them. Lots of ways to implement them, too, which is the flexibility you're pointing to.
Don Wilde
"There's more than one level to any answer."