The following works because you can write table formulas with elisp code. Notice the ' (quote) before the s-exp.
| # | B | C |
|---+---+---|
| 1 | 2 | 3 |
| 1 | 2 | 3 |
#+TBLFM: @1$1='(format "#")
Evaluable (info "(org) Formula syntax for Lisp") for more information.