Using JSON data structures seem to be possible in basic way :
@startuml
!$partlen=2
!$data={"partlen": "2", "participants" :[
{"name": "XYZ" ,"as": "xyz"},
{"name": "RST" ,"as": "rst"},
{"name": "UVW" ,"as": "uvw"}
]
}
'!$partlen=$data.partlen
!$i=0
:a;
:$i;
:$partlen;
':$data.participants[$i].name as $data.participants[$i].as;
:$data.participants[0].name as $data.participants[0].as;
:$data.participants[1].name as $data.participants[1].as;
:$data.participants[2].name as $data.participants[2].as;
:b;
@enduml
but using parameters to access the content produces errors, using integers is ok.
Its this a bug or a missing enhancement