Hi Mark,
You're quite right of course. Currently in the UK at least there is very little in the way of a registration structure for F16. Wouter tells me that a formal structure will be in place soon.
I guess I was referring in general terms to the measurement/registration process administered by national sailing associations and ISAF. For example you buy a 49'er with a builders cert, get it measured, apply to your national sailing authority with those two documents and get your national sail number.
Or perhaps you buy a used Tornado from abroad (hopefully complete with documents) and you apply to your national authority and are issued with your new national sail number which you then put on your sail to replace the 'foreign' one.
Or, if you want, you talk to no-one official and you stick whatever the hell you want up there -

It'd be a talking point......