Attempt to create trigger with expression:

{lld macro test:dummy.echo[{$ECHO}].count(10m,{$MACRO:with , character})}=20

results in error:

Incorrect trigger function "count(10m,{$MACRO:with , character})" provided in expression. Invalid third parameter.

I suspect that server wouldn't be happy to see such trigger function parameters either, because get_param_dyn() we use there is not aware of context based macros too.