In thread and frame format strings, it is now allowed to use Python functions to generate part or all of the output text
Specifically, the ${target ${process ${thread and ${frame specifiers have been extended to allow a subkeyword .script:<fctName> (e.g. ${frame.script:FooFunction})
The functions are prototyped as

def FooFunction(Object,unused)

where object is of the respective SB-type (SBTarget for target.script, ... and so on)

This has not been implemented for ${var because it would be akin to a Python summary which is already well-defined in LLDB