Ive used the Helo FARPs too with the harrier loaded to about 26,000 - 28,000 and it takes off ok.... just need to nurse it up to speed. I back the jet up to one side of the pad (before refuel/rearm) and get a small running start as well.
Personally I would think a combat jet loaded with weps would not be able to perform a STOL on grass unless it was compacted a lot