At 20km they don't have to be there for long.
I remember a "drop a laminated token in a bucket someone is holding at the side of the road" control within the first 10km of the one 200[1] many years ago. Worked well and the person was probably only standing at the side of the road for 15 minutes or so, easily free to be able to man a control later on in the ride.
1. Upper Thames or maybe it was the SBWW. 10 years ago at least.
Yes, Upper Thames, 2004 and 2005. It didn't really work that well - too many riders seemed confused by the concept of taking a token at the start that they dropped in a bucket a few km down the road as they rode past!
Many riders have told me that when I moved the start/arrivee and dropped from 3 controls to two it was a much better event. The loss of a big hill in the final 10k was part of that (it's still in the event, but near the start instead), but lots expressed a preference for the two (almost exactly equally spaced along the route) controls. 70km gives a far better chance of a late starter reaching the first control "in time". Always supposing that the start is still manned for their late departure.
Given the availability of food outlets in England and (most of) Wales, I agree with Wilkyboy's comments above, but rather different criteria might apply in much of Scotland.
Given the OP's statement that they are organising a mandatory route event the number of infos is irrelevant. There are plenty of threads here about them anyway.