I guess you can represent a FOR loop in an activity diagram using a repeat while loop, eg
@startuml
start
repeat :For i=1 to 10;
:print i;
repeat while (next i)
stop
@enduml
@boldandbusted - I don't understand your question "can I use for loops across the PlantUML language, irrespective of object type?". I mean loops only really make sense when you are drawing an activity diagram... Did you have a diagram type in mind?