If you wanted to have consts outside of math expression, you can make command instead. Yes, you could be careful to not reassign variables. Here’s an approach:
foo:
m "$0_A:42" # Const here
e $0_A
e $$0_A
e $${$0_A} # You need to do this to access your const
$0_A=9
1,1,1,1,""$${$0_A}"" # Image value is the const
1,1,1,1,${$0_A}
um $0_A # Unassign command associated with const