Howsabout a special silver or silver/gold coin as a reward/award?
Yes, I know I'm biased quite heavily in this suggestion, seeing as I gobbled up a ton of the coins just to try to get a full series.
But, that's what made it extremely FUN! Trying to find the coins so you could get a complete set. That's what us actual collectors want... we strive to complete a full set of "something". Be it a numbered set of all 1's, or a full set of MS-9000 graded, or whatever.
I could give a crap about most of the alts (and yeah, WTH is Coin2???). I wanted them primarily so I could complete something. I'm not really knocking your idea of "hey, this is part of history", I agree. But there's obviously a larger alterior motive as well...
I'd say make a run of the most popular coins - Bitcoin, Litecoin, Namecoin - with different true PM options. Make some all silver, make some silver/gold gilded, make some different weights (.5oz and 1oz maybe), and have some fun with an "awards" system. If you have only a couple 3-coin silver sets made, and you need to complete a full set of whatever in order to get one of them, it should drive interest, and trading, to get them. Number them, make them special colors or special designs to set them apart from the normal ones.
You kinda did this with your offer of a free set of the 2nd series to the first one to complete a full gold-plated set. Take it to the next level.
That's my idea for making it a true collectable, and to drive people to want to be able to buy them and collect them.