This looks like a mix of ZBXNEXT-4087, ZBXNEXT-4109 and request for advanced JSON path syntax which is not provided by ZBXNEXT-3863.

Another example where this would be very helpful:
a dependent item prototype where the masterkey (which should not necessarily be an item prototype itself ZBXNEXT-4200) delivers a csv list:

# type,n1,n2,n3,n4
a,0,0,1,0
b,0,0,2,0
c,1,0,1,0

The LLD macro {#TYPE} with values ['a','b','c'] and for item {#TYPE} n3 we would have a preprocessing like this:

Regular expression; ^{#TYPE}([^,]*,){3}([^,]*).*; \2

I have a similar but much simpler use case - I need to use a returned {#JSONPATH} LLD macro value in the dependent item JSON pre-processor but it doesn't seem to work?

Available in pre-4.0.0alpha7 (trunk) r80928.

Using Zabbix 4.0.0beta2, I've this issue

 
Item preprocessing step #1 failed: cannot extract value from json by path "$[?(@.serial == '{HOST.HOST}')].status": unsupported character in json path starting with: "[?(@.serial == '{HOST.HOST}')].status"

using an HTTP item that returns a Json:

[
    {
        "name": "xxx",
        "serial": "xxx",
        "status": "online"
    }
]

alxtom, such JSONpath expression is not currently supported. See ZBXNEXT-4502