Thanks also for this question.

I thought carefully on it this afternoon.  I know parts o the wiki quite well, especially the taging schemes, have used it continuously as reference before I used new keys and even have amended parts of the wiki on the seamark tagging scheme. But in this case, if you don't know the existance of the import guideline, you would hardly start searching it in the wiki.

I have also gained quite some experience with OSM work in JOSM, applied the presets carefully and always checked carfully the validation results before uploading. Also in this case of the large seagrass import I made much effort to check, address and solve all validation errors and 95% of the warnings. 

From my perspective I can name two things, that would certainly have stopped me importing a very large set of already very carefully and well JOSM validated data:

1. A dedicated upload warning window or explicit well described validation warning for the large amount of data in the upload. Such warning should best include a direct link to the required (for me unknown) import guidelines and any other procedures to be checked in the wiki. There the requirement to open a website, ask for community review, use a special user name, etc. would have popped up. I know some other cases, where warning windows in JOSM included a link to further explanation in a special wiki website.

2.  A warning or error messeage during the upload procedure requesting  in the upload description for the large amount of data to include as a value the reference to the import plan website. Again this request to provide something, that I did not know of, would have started me to think twice and search for it in the wiki.

The question is: Does OSM experience tell a reasonable size of an upload, above one can most likely assume, that the import guideline should be applied. Then a combination of both would work best: 
1. a JOSM validation error or validation warning with a link to the 'import Guideline' (https://wiki.openstreetmap.org/wiki/Import/Guidelines) and a request to use a seperate username for such large upload and 
2. the requirement in the JOSM upload to link the created wiki page outlining the details of the import plan (Step 3.2 in the guideline), e.g. next to and in addition to the source information. 

I hope this idea is helpful and that this laboursome experience can perhaps add some value for the future of OSM.
