Let me know if I'm misunderstanding the proposal.  Are you proposing a non-standard ASDL extension to that won't work with existing ASDL parsers?  I thought the entire point of ASDL is that it was standard, portable, and had a fixed number of types easily implemented in many languages.

FWIW, Here are some of my notes from a compiler project I worked on a while back:
# ASDL's seven builtin types are:
# - identifier, int, string, bytes, object, singleton, constant
# - singleton: None, True or False
# - constant can be None, whereas None means "no value" for object
#
# Resources for ASDL:
# https://www.usenix.org/legacy/publications/library/proceedings/dsl97/full_papers/wang/wang.pdf
# https://eli.thegreenplace.net/2014/06/04/using-asdl-to-describe-asts-in-compilers
# https://www.oilshell.org/blog/2016/12/11.html