I think it rings a vague bell, but it is certainly not in common usage.
My guess would be that it derives not from the advertising/marketing industry itself, but from 1950s left-wing critiques of consumer capitalism. It is a bit of quasi-Marxist jargon that may have had some currency for a while, but never caught on very widely.
Never heard of it.
I wouldn’t disagree, but it’s not as if the reviewer coined the word. Most large dictionaries contain it, and it’s older than most of us here. I was soliciting comments based on how the reviewer used it.
But we can talk about breasts or tampon management techniques if you think those are more fruitful lines of discussion. 
Eh?
Other threads here that would seem to rank below a legitimate etymological/linguistic inquiry. But that’s not important right now.
I’m not from the UK, but interestingly I just saw this word in situ. Under one of those “see what Ellen DeGeneres lied to her fans about” ads was a mini, floating pop-up that said “Want admass for free? Click here”. I’m paraphrasing. I didn’t think to try to snag the URL of the mini, floating pop-up, if indeed you can do that in the first place.
I think that reviewer intended the word ‘adlib’, or possibly ‘madness’ and something went wrong between brain and keyboard (or possibly just in brain)
Never heard of it.
That’s a different topic altogether.
A quick Google Ngram (British English) suggests “admass” peaked around 1966 and has steadily declined ever since. (Insert your own “England football team” joke here, satire fans.)
In fact, comparing it with “mass marketing” as a roughly equivalent term, you can see that admass caught on faster initially, but never became as widespread.
Never thought to Ngram it - not a tool that’s made it into my daily toolbox, for some reason. Thanks.
I’ve never heard “admass” used before this thread, although I would have guessed its meaning fairly accurately.
I agree 100%. The term “admass” as defined in the OP’s link makes no sense in the phrase “They plough a completely original furrow of snatched conversation, admass and inane banter…”, so I’m sure it’s a mistake.
[As an aside, I also question the IMDB reviewer’s assertion that Monty Python’s “painful varsity guff” “isn’t what Fry and Laurie do at all”. I am a fan of both of the above-named teams, but to suggest that Fry & Laurie (both Cambridge, heavily involved in Footlights) are distinctly less “varsity” than Monty Python (Cambridge x 3, Oxford x 2, and one exception) is belied by the evidence, IMHO.]
Even I’d agree here. “Ad-libs” is almost certainly what the reviewer meant.
It’s odd that such an odd and almost-right word would appear and lead me to the confirmation of its valid origins and at least sometime use. Possibly a spell-check error? I wonder how many spall chuckers have such an obscure word in their files.