Alright! Just found an existing tag with precedent for this :)

ref:US:NID is used for the National Inventory of Dams. The IDs in that database are already prefixed with two-letter state code e.g. OK01147 or CO01660S001. In the case of NBI data, the state code (FIPS PUB 5-2) is separate from the state-unique structure number, I think the clearest option is to make this explicit:

    ref:US:NBI:state_code=01
    ref:US:NBI:structure_number=044B00032N

Making things slightly more complex is that NBI is transitioning to a new specification (SNBI) with a new schema. I suppose when that happens some years in the future, the tags could be replaced/transitioned to ref:US:SNBI with the new field names. Even so, state code and bridge number will be the most relevant tags.

Oh and about the licensing questions: this has been clarified on https://hub.arcgis.com/datasets/geoplatform::national-bridge-inventory-nbi/about and other locations with a notice such as the following

This NTAD dataset is a work of the United States government as defined in 17 U.S.C. § 101 and as such are not protected by any U.S. copyrights. This work is available for unrestricted public use.

An obstacle to the proposed approach is that NBI Structure Number is only unique at the state level. There is no format specified, any 15-digit alphanumeric sequence will do.

To make sense of it, one would either need state-specific tags, e.g. ref:US-IN:nbi=1000149 and ref:US-KY:nbi=044B00032N, or the parser would need to use both state and structure number to uniquely identify a bridge. I suppose there may be edge cases where the maintaining state is not obvious from the position of the way/node(s) in OSM, so it should be separate metadata. I’m not extremely happy with either idea :/

Thank you for the comments :)

@lyx I agree, after I posted this, I did more research on ref, and it seems more conventional and less confusing to indicate the country.

As for licensing, as a published work of the US government, I believe this data passes into the public domain. I’ll be happy to contact them and ask for clarification!

@Tordanik Indeed, I examined the taginfo for bridge:structure and it seems used exclusively, structure by itself indicates something different.