According to The ACBL Alert Pamphlet
Except for those doubles with highly unusual or unexpected meanings, doubles do not require an Alert.
I do wish they had provided more examples in common situations like this one (1NT-X).
A natural, penalty double, while uncommon opposite ACBL-style strong NTs, is hardly "highly unusual or unexpected." Definately not Alertable.
While an argument could be made that the double of 1NT is so frequently conventional that it is in nature "self-Alerting" (the idea behind the rule), it's probably just safer (especially if the doubler is one of the many ACBL players that ignore the "two complete and identical CCs *on the table, for the opponents' view*") to Alert any conventional double.
Most "takeout" doubles of NT openings are conventional, having more of a meaning than simply "bid something, partner", so I (and I'd) Alert them, be it "one-suited", "one-suited, good", "11+, spades and another, 4-4 or better", or whatever.
I have played against a pair that played double of 1NT as "4-suit takeout". I don't understand it - how could you possibly have a suitable hand? - and they really did take it out always (to their detriment at least 3 of the 4 times it came up during the match). I would say this is "highly unusual", and Alert it (maybe the opponents' brains will break on the explanation and they'll screw up the auction).
If you want an exact ruling, I'd email The Source
, or see if you can get an official response from the ACBL's Alert forum.
However, in practice, no matter what the legalities, if you Alert any non-penalty-oriented double, and any double intended as penalty that has any distributional or max strength requirements, you'll win in the long run, because nobody will call the TD on you "failure to Alert".