Just to be clear...

Is there any possible way for these coupons to have value again? I mean outside of having the exact same event again.

For instance, if we have another event that involved Caravan Joe, will he allow trading in the coupons?

Other than a souvenir/nostalgia purposes, it's pretty much useless. Though given the popularity of Vocaloids, there may be some degree of having the coupons for use again, unless they get repurposed or something. But as far has reusing the coupon where joe is available, I'd say no. You wouldn't be able to trade anything for them, making their value worthless.