The most prominent one (by looking at tags and actual use in maps) is the seamark:* namespace - although I saw that this was never voted on. 
On other tags that occasionally require long values like inscription and destination/direction it's also pretty common (but not approved) to use the colon as separator.

I personally don't care if we start with 1 or 2 - as long as the numbers are correctly sorted in cases where it matters.