i would use a couple of observation fields to allow you to filter for selected observations. one would be something like herbarium code and another would be collection number.
while you haven’t yet put your observation specimen into the herbarium yet, you could set herbarium code = some temporary value like “herbarium A work in progress”, and once you’re done you could just drop the “work in progress” and set collection number to whatever the collection number ends up being.
(alternatively, you could just use a single collection number observation field, and set it to some dummy value until you’re got a collection number for it.)
then you would be able to filter by observation field and value, looking for only observations where herbarium code = “herbarium A work in progress” (or collection number = [dummy value]).
here’s an example where i’m going to export only my own Loggerhead Shrike observations from Brazoria National Wildlife Refuge where banded = yes: https://www.inaturalist.org/observations/export?verifiable=any&place_id=119232&taxon_id=12024&user_id=pisum&field%3ABanded=yes
in the future, you might be able to filter for cases where a particular observation field has not been added to an observation, and in that situation you could just look for observations without a collection number observation field. but until then, you need to explicitly set some observation field = to a temporary or dummy value so that you can filter on it.