My 2 nem:

An auction doesn't seem completely fair to me. Nem has quite a bit of newbies and 'friends of' who can't freely sling around bitcoin. To me it seems a NEM stake is now already worth more than the original buy-in, and you shouldn't be distributing an egalitarian crypto on the basis of who can afford to pay the most.

In the hypothetical case of 300 spare stakes here's how I would proceed:

290 go to the people on the waiting list through a lottery. Put those at 0.1 BTC or something to keep things straight for the people who got in earlier.

Then, refund the people that somehow thought it was a wise idea to send money while on the reserve list.

Then, throw a public auction for the last 10 stakes, and send the proceeds to charity.

EDIT: in the event that everybody on the waiting list gets a stake I agree with the above poster: auction off the remaining stakes for charity.