For other readers, I provided more guidance on using “live” metadata in /construction/preprocess metadata on a different topic.
I also put up a PR to improve the wording on the spec around this point.
This testing tool improvement will allow for providing any “non-live” metadata to ConstructionPreprocessRequest.Metadata that may be relevant for transaction construction (like this “ttl” you mentioned).
I’m thinking most of the gains here could be accomplished with a custom DSL on top of these low-level operations. If we find this isn’t the case, having a “custom” op that just calls out to an HTTP endpoint may be a good compromise.