Just to clarify, I wasn't implying to allow people to mail a check or money order for a dining reservation. More thinking along the lines of mandatory credit card holds (sorry for those who don't use them) which could include debit cards and maybe a PayPal option for those using online booking.
But that brings us back to: If Disney doesn't require a CC to hold a room, why would they require one to make a dinner reservation? Because now you could be excluding a segment of your resort guests from making dining reservations...when you've spent years building up your resort to, in a nice way, hold people hostage on property (with ME, ADR's, Theme park transportation, etc all meaning you can hop off your plane, go to WDW, never leave the resort, and hop back on to your bus to the airport).
Considering reservations are already tied to a person, it wouldn't be impossible or out of line to imagine that the ticket/guest ID assigned to the reservation could have a credit on the account when the person checked in. This feature could also be a benefit too, people could pre-pay part of their meals if paying OOP.
Again, though...not everyone making ADR's is a Resort guest.
And it's unlikely that Disney would be willing to adopt the "sorry for those that don't have them" mentality, given it could exclude a decent number of their guests...which would create a CS nightmare for them. It could also cost them revenue and they're NOT going to adopt any solution that could, even potentially, do that.
It's a fine tightrope to walk. Any solution has to solve the problem, contain costs/resources for Disney, AND not create enough of a "negative guest experience" that it creates outcry. And make no mistake, forcing a non-resort guest to have a CC to make a reservation WOULD.
As for the credit system, yes, it's possible. But it's both inefficient and would create the possibility of HUGE CS issues when/if a deposit was lost, misplaced, miskeyed, etc. And, again, you're back to the rather large number of transactions that would be required by your proposal. Given the number and nature of those transactions, you'd have to expect a certain error ratio...and it would be rather difficult to resolve them in that setting, all things considered.
There just isn't a great system to impose "deposits" in order to secure ADR's that's both fair, efficient, and cost-effective. I understand why people want them, but I doubt we'll ever see them. They're just not as "simple" as people suspect.