Quote from Mr_Lonely :

We share the skins over skype in highres ofc.. :P Its always with friends. No random date. lol

If you upload a blank skin of the same name, then whoever you shared the proper one with will see you skin, they will not redownload the blank one.

But why go through all that tho ? Just seems unnessesary.
Its a good idea for public servers, dont see it doing any good for private partys.

Fair point. Shouldn't even use up much dev time (useful since you won't get many people clamouring for this particular fix!).
I had managed to forget that the fetch from LFSW only happens if the skins folder doesn't already have a match

Wasn't this changed originally due to some client lag issue due to missing skins, i.e. there was a technical reason for it?

Yes

That does ring a bell.
However, that's a truly lame reason to make rules for skin uploading. The real answer is to fix the flaw that causes the lag in the first place (which isn't gone btw)...