I am very envious. I want to be a Numbers Count teacher! My previous school had one who did a great job helping to fill the Maths gaps for year 2 (mostly) pupils. She loved her job. Because she was in school, she would sometimes cover a class if necessary, so she still got to do some class teaching too (although that was because she was there and willing and part of the team, rather than because it was part of the job)
I agree that it would be much more family friendly than being a class teacher. I also agree that it would be great to be able to work with children on a 1:1 basis - you could make a huge difference in a small amount of time. The training is very good too, from what I saw (and I'm a MaST specialist).
As for how long term it is, who knows? I would assume that as long as schools are getting additional funding like pupil premium or deprivation funding or whatever your LA prioritises, there will be schools who choose to spend their money on a very effective Maths intervention.
Being out of the classroom a problem for the future? Who knows? It might just be the move that saves your sanity and so means you are able to stay in teaching rather than doing what so many people seem to be doing at the moment, and cracking under the strain. I was recently involved in short listing for a job at my (lovely)school. We had all of 3 applicants, none of whom were currently in a permanent position - 2 had had longer breaks from the state sector than the other (private school work/bringing up family/supply was the reason for the break). The big concern was that the candidates wouldn't be up to speed with current assessment requirements as things have moved on so much in the past few years. If you're working in a school and attending staff meetings (where relevant to you - my former colleague wasn't at every meeting but she was at inset and some after school meetings) then you'll be able to talk your way out of that problem. Also the assessments used in Numbers Count are well research and rigorous.
I would have thought you could sell your Numbers Count experience as a bonus rather than a drawback at future job interviews. And just think - you might even manage some kind of home life. Go for it. And if you don't, let me know where so I can apply...