Wikipedia has a nice entry on this. In short, "homoiconicity is where a program's source code is written as a basic data structure that the programming language knows how to access."
TRAC is lots of fun. I read about it in Nelson's book, Computer Lib/Dreams, when I was a freshman at Illinois. That spring, my Dad bought an Altair and I wrote a version of Trac for it, in assembly. Had support for bignum arithmetic, in ASCII :-)
Later, when I learned about tail recursion, I was happy to figure out that my implementation was indeed properly tail recursive.
You cannot simply expose the compiler/AST data structure and call your language homoiconic because the text the user enters and the resulting AST are not isomorphic which is a necessary precondition for homoiconicism.
I may be missing something in what you said though, so please do let me know.
def x :Int := 42 # evaluated statement
def ast := m`def x :Int := 42` # quasi-quoted Monte fragment
eval(ast, safeScope) # easy evaluation
def m`def @lhs := @rhs` := ast # pattern-matching!
[lhs, rhs] # [mpatt`x :Int`, m`42`]