...secret controls .....................they only work if the last or nearly last info control is a secret control; if it's early on riders know the secret controller has been and gone and can happily ignore the others, as the secret controller will have to spend too long at each,
Yes, I can see that, and I doubt it can be totally overcome. However, I was envisaging a scenario where one secret control could, perhaps, replace several info controls in, say, one segment, leaving a combination of other controls elsewhere (infos, receipts, stamps and secrets) . Also, as has been mooted above, the idea of excess several secret control boxes on the brevet card, so that the rider doesn't actually know how many there are, going some way to resolving your concerns had occurred to me.
As for manning all of this, let's say the ride potentially had 3 manned controls and 8 infos. It is likely that the 3 manned locations involve retail establishments so they might be turned into 'receipt' controls freeing up 3 controllers who then go out and man 3 secret controls to replace some, or all, of the info controls. The outcome, if this were feasible, would be a drop from 11 to 6 controls. It may not suit every event, but probably merits some consideration.