What is the real source of the strings you are processing? (If you are echoing constant strings into awk, it would make much more sense to remove the awk and just echo the string you want.)
Again, if you can modify the input string as well as the ERE, why are you using awk to modify it instead of just changing the echo to begin with.

Furthermore, if you are always trying to remove a fixed string from the end of an input line, use match() to find the fixed string instead of worrying about modifying special characters in an extended regular expression.

If you really need to use an ERE to split fields, give us a clear specification of what characters might be in the input that are special in an ERE.

For the specific examples your provided you could try:

Code:

echo /a/b/c+/d | awk -F"/c[+]/d$" '{print $1}'
/a/b

and:

Code:

echo /a/b/c\+/d | awk -F"/c[\][+]/d$" '{print $1}'
/a/b

Also note that using echo to feed data that might start with a minus sign or might contain a backslash character can produce radically different output depending on what shell you're using and on what system you're using when you use that shell.