I agree with your interpretation Giles
Clearly not balanced so could easily influence the opps bidding and or play

I interpret the manual as "12-14, could contain a void" should be announced, not alerted.

I'm basing this on:

1. Tradition. In my admittedly limited experience, players who play unbalanced 1NT tend to announce e.g. "12-14, could contain a 5-card major/6-card minor/void". (Just because this is done by some players doesn't mean it's the right thing to do, of course.)
2. Pedantism. The phrase "No further announcement is required" in the notes under the 1NT announcement on page D60 leads me to assume that in the event of any other, unnatural, distribution, it should be "further announced".

For a natural 1NT opening bid:  "12 to 14" (or the appropriate range) Note: A 1NT opening bid that could contain a single (an unbalanced hand) is considered a natural 1NT opening bid if the partnership are prepared to play in 1NT or be transferred to their singleton suit and play there. It is alerted by announcing as above and no further announcement is required.

I would interpret that the same would apply for a partnership agreement where the hand could contain a void as the hand still meets the definition of an unbalanced hand.  If a natural no trump could not contain a void then the general definition of an unbalanced hand would not have been referenced in the note. The key is what are their partnership agreements if transferred to the void. If they take no further action then it should be announced "12 to 14" or the appropriate range.

Unbalanced : A hand that is not balanced nor semi-balanced.

In theory,  they could have agreements to open 1NT with two voids and no further announcement would be required provided they are prepared to play in 1NT and to be transferred into one of their void suits.  

If they have agreements to take further action when their partner transfers to their void suit then the bid should be alerted rather than a further announcement being made in my opinion to ensure no unauthorized information is transferred.

In a related matter, their system card should be changed to RED, if not already done so, as having a partnership agreement to open an unbalanced 1NT does not comply with requirements for a GREEN or BLUE system

It definitely required an alert and a full explanation if asked

If it was "any shape 8-11 (maybe exculding some normal pre-empts)" it doesn't in itself appear to be a HUM or Brown sticker (which surprised me) so it would be allowed at any level above Junior (can anyone confirm or refute this?)

If their methods were balanced 8-11 then it was a psyche
Which is legal subject to the general restrictions on psyches

Seems normal to me!

But on a serious note, I think anyone who would systematic be opening this 1NT would be alerting this. If you played something like 1NT 8-11 unbal then this is well within range.

1NT in Junior/Intermediate/Restricted 8B/Multigrade 8B tournaments are all required to be 11+hcp and balanced (ish) under the current regulations.

As a funny aside, North Shore have a rule that 1NT must not contain a singleton. This doesn't contain "a" singleton it contains two so passes that test.