As a former professional philatelist(Stamp Dealer), I dealt with a ton of first day covers and other things of this nature. Typically the people putting together a new first day issue, or an event, create a special envelope with a cachet. This is an image of some sort generally on the left portion of the cover. There were even companies, like Artcraft and Fleetwood who were in the business of making these, and each would be different yet the same theme as the stamp that was being released as a first day cover. For what you guys are talking about, it is an "Event Cover", so the image or illustration must be created for the envelope. This is the major prep work not counting setting something up with the Post Office. I am not sure how official these things really are. If the PO is involved them it could be legit, but not sure what the legalities are here. They may say its okay to produce your own "Event Cover" and use your own cancellation on it as you see fit. You would need to google this and follow-up from there.
The better looking the cachet, the more that will sell. Think a Gorgeous color painting with the new car in the consist of a period train. When 168 comes out a great image of that engine should be on the cachet. Now to put a wet blanket on this. It is my experience these things are fine as collectibles, but really have very little collector value down the road. Remember they are specialized "Event Covers". Resale value is minimal. Even first day covers from the 50's are still really cheap as lots were sold. When I worked for my dad we once bought a garage full of covers in North Carolina and filled the truck camper (67 or 68). Dad sold them for something like $10.00 for 50 different, or something like that.
Now what you could do is put some sort of incentive item/s inside the envelope, that creates some worthwhile value. For instance a nice set of 4 John West color photos, or something like that as an example.
Also I would see how many are really interested in something like this, before spending a lot of time and money to put things in motion. You wouldn't want to have 80-90% of the inventory left afterwards!!!
Just some food for thought!
By the way I have no association with that business whatsoever today!
Greg Scholl