What you propose, Ryan, sounds great. I actually came across a similar piece elsewhere on the internet some months ago, but wasn’t motivated to buy it because it was heavy for what it was (didn’t have attention to lightness you lavish on your own products).
However, that other item was really only a hood (plus the shoulder cover and underarm straps you describe for your new product). From your description above, your new product also sounds like a hood although you call it a balaclava – in the specs you didn’t refer to any front face cover – or am I reading it wrong?
I have also tried the GoLite snowcap, but (apart from getting a large enough size) it has two problems – still leaves the face exposed, with all that heat loss; and doesn’t cover the neck well.
Although I sleep in a “tube” style mummy bag – a GoLite without a mummy hood – I’d welcome an auxiliary piece like the insulated balaclava you describe, to extend the comfort range of my present system. I can also see value in it when using an “elephant’s foot” half-bag (I have the Wild Things Gear half-bag) in combination with a lightweight parka, or even with a Patagonia micropuff sweater. Your piece would definitely fit a missing part of the niche.