it would be possible to write a gcc tree to C99 "backend" - probably post
gimplification and replacing the call to lowering the trees with the new
C99 "backend".
I am not sure that this would produce readable C output. If you let the GCC middle infrastructure do transformations on the syntax tree, there will most likely be significant changes relative to the original input. The resulting code would likely be very low level and thus difficult to read and debug. For example it would probably split up expressions into smaller and smaller sub-expressions. It may even turn WHILE and REPEAT and FOR loops into label and goto based equivalents.
I still consider the approach of transforming the syntax tree in its original form from Modula-2 to C as close as possible and as often as possible in a 1:1 manner to be superior.
regards
benjamin