Use quoteless strings, or tc(utf, 22) to concatenate a doublequote to your loop body string, or use ' or another placeholder and replace it using tc(reg):
$fl(0, 9, "i+1", tc(reg, "'$' + i", "'", tc(utf, 22)), " ")$
This example prepends $ to i, and $ has to be quoted - so I put it in ' and then replace those ' with " (which is returned from tc(utf, 22) as we don’t have a way to escape doublequotes).
I think the solution to this would be to add a way to escape doublequotes with for example a backslash so you could just do "\"$\" + i".