I like it when someone finds a hole in the rules. This looks like one of them. Interestingly, the rule is the same for RB. Nothing seems to prohibit purchase of Cloak ? as being tied in with the purchase of reserves; otherwise, the table should explicitly state that cloaked dummies need to be purchased with reserves only -- and it doesn't. If some Q/A answer comes back justifying this is not allowed, after all these years, well, that would be typical but also a shame.
This is a case where no rule should be added that isn't written. Any case relevant to cloak dummies in the night rules is only relevant to the extent of E1.4; that is, cloak ? must be of a nationality color not in play. This has been true in RB and should remain the same.
I imagine that this ploy would work as a kill zone trap if you could get the enemy to think that the front lines are weaker than they really are by having a mass of dummy reserve units represent a substantial investment in CPP, combined with a lot of HIP of real units. Maybe fun, risky, and tough to pull off.